xfs: detect incorrect gaps in rmap btree

Following in the theme of the last two patchsets, this one strengthens
the rmap btree record checking so that scrub can count the number of
space records that map to a given owner and that do not map to a given
owner.  This enables us to determine exclusive ownership of space that
can't be shared.

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