xfs: accumulate iextent records when checking bmap

Currently, the bmap scrubber checks file fork mappings individually.  In
the case that the file uses multiple mappings to a single contiguous
piece of space, the scrubber repeatedly locks the AG to check the
existence of a reverse mapping that overlaps this file mapping.  If the
reverse mapping starts before or ends after the mapping we're checking,
it will also crawl around in the bmbt checking correspondence for
adjacent extents.

This is not very time efficient because it does the crawling while
holding the AGF buffer, and checks the middle mappings multiple times.
Instead, create a custom iextent record iterator function that combines
multiple adjacent allocated mappings into one large incore bmbt record.
This is feasible because the incore bmbt record length is 64-bits wide.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

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