xfs: merge bmap records for faster scrubs [v24.5]

I started looking into performance problems with the data fork scrubber
in generic/333, and noticed a few things that needed improving.  First,
due to design reasons, it's possible for file forks btrees to contain
multiple contiguous mappings to the same physical space.  Instead of
checking each ondisk mapping individually, it's much faster to combine
them when possible and check the combined mapping because that's fewer
trips through the rmap btree, and we can drop this check-around
behavior that it does when an rmapbt lookup produces a record that
starts before or ends after a particular bmbt mapping.

Second, I noticed that the bmbt scrubber decides to walk every reverse
mapping in the filesystem if the file fork is in btree format.  This is
very costly, and only necessary if the inode repair code had to zap a
fork to convince iget to work.  Constraining the full-rmap scan to this
one case means we can skip it for normal files, which drives the runtime
of this test from 8 hours down to 45 minutes (observed with realtime
reflink and rebuild-all mode.)

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
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