fuse mount failure when type contains '.'

`util-linux` seems to fail to handle a use case that users like me seem to
be hitting with some frequency. The issue is around
https://github.com/util-linux/util-linux/blob/86b6684e7a215a0608bd130371bd7b3faae67aca/libmount/src/context.c#L2115-L2121
and seems rooted in the fact that `util-linux` operates on `type` where
type is always of the form <a>.<b> but where <b> is apparently assumed to
not contain Unicode U+002E (ASCII 2E): '.' ("dotless"). I say it "appears
to assume" this since the logic which appears to remove the subtype does so
by using `strrchr`, which addresses only the last period in the `helper`
string. If the `subtype`, itself, has a U+002E character then this won't
remove the subtype.

As an example, if `helper` looks like
`path/name.type./subtype/path/with/a.period` then the modified `helper`
after `strrchr` will look like `path/name.type./subtype/path/with/a`
instead of the apparently-intended `path/name.type`.

This crops up for users like me because  I use NixOS which is a store-based
Linux operating system using paths like:
```
$ readlink -f $(which s3fs)

/nix/store/xwbx0fbg65ml2qjl86p9p2w58kghqn8r-s3fs-fuse-1.94/bin/s3fs
```
(So, paths like `/nix/store/<hash>-<name>-<version>/bin/<cmd>`). `version`
is usually a dot-delimited string like `1.23`.

I've generated a patch which seems to ameliorate this behavior. It's
attached (sorry if attachments are not the way to go with this mailing
list, specifically, or mailing lists, generally - this is my first time
submitting a patch to a mailing list). The logic is simple, but it
basically iterates through all possible substrings according to the number
of U+002E characters in the `type` string. It's a more generic form of the
logic already present, but it's a little heavy-handed. Happy with any and
all changes which preserve the apparently-corrected behavior.

Please let me know if I should make any changes or if a change like this
won't be accepted for some reason or if I'm misunderstanding the
problem/solution.

Thank you!

Cf.
1. https://discourse.nixos.org/t/how-to-setup-s3fs-mount/6283/5
2. https://github.com/NixOS/nixpkgs/issues/46529#issuecomment-655536831

-- John Rinehart

diff --git a/libmount/src/context.c b/libmount/src/context.c
index d38e3671f..411f98f3f 100644
--- a/libmount/src/context.c
+++ b/libmount/src/context.c
@@ -2097,6 +2097,11 @@ int mnt_context_prepare_helper(struct libmnt_context *cxt, const char *name,
 	if (!ns_old)
 		return -MNT_ERR_NAMESPACE;
 
+	/* Record number of '.' in `type` so we can try a few things later. */
+	int dot_count = 0;
+	for (int i=0; type[i]; i++)
+		dot_count += (type[i] == '.');
+
 	/* Ignore errors when search in $PATH and do not modify @rc
 	 */
 	path = strtok_r(search_path, ":", &p);
@@ -2112,8 +2117,8 @@ int mnt_context_prepare_helper(struct libmnt_context *cxt, const char *name,
 			continue;
 
 		found = mnt_is_path(helper);
-		if (!found && strchr(type, '.')) {
-			/* If type ends with ".subtype" try without it */
+		/* If type contains '.' then try without them. */
+		for (int cnt = dot_count; !found && cnt; --cnt) {
 			char *hs = strrchr(helper, '.');
 			if (hs)
 				*hs = '\0';