don't build bios/contexts over multiple iomaps v2

Hi all,

this patch changes how iomap submits bios for reads.  The old behavior
to build up bios across iomap was already considered problematic for
a while, but we now ran into a erofs bug because of it, so it's time
to finally fix it.

It would be great to get the fix into 7.2 as the fixed bug can be
triggered by users.

Changes since v1:
 - don't submit fuse context after each iteration
 - consolidate some code to support the above
 - fix a bug in the fs PI support found while doing the above