make xfs sparse-warning free

Hi all,

this series isn't really a series, but a collection of two very different
patches toward the result of having no sparse warnings for fs/xfs/.

Patch 1 adds a cond_lock annotation to the lockref code.  This also fixes
warnings (but resurfaces new ones) in erofs and gfs2.

Patch 2 moves some XFS code around to help the lock context tracking. 
I actually think this improves the code, so I think this should go into
the XFS tree.

Patch 3 duplicates some XFS code to work around the lock context tracking,
but I think it is pretty silly.  Maybe it's a good example to help improve
this code in sparse?  It would not be horrible to apply given how little
code it duplicates, but a fix in sparse would be much nicer.

The kernel MAINTAINERS still list Luc as sparse maintainer, but sparse
itself lists Chris again.  Do we need to update the kernel MAINTAINERS
file, or are those separate roles?