xfs: don't call xchk_bmap_check_rmaps for btree-format file forks

xfs: don't call xchk_bmap_check_rmaps for btree-format file forks

The logic at the end of xchk_bmap_want_check_rmaps tries to detect a
file fork that has been zapped by what will become the online inode
repair code.  Zapped forks are in FMT_EXTENTS with zero extents, and
some sort of hint that there's supposed to be data somewhere in the
filesystem.

Unfortunately, the inverted logic here is confusing and has the effect
that we always call xchk_bmap_check_rmaps for FMT_BTREE forks.  This is
horribly inefficient and unnecessary, so invert the logic to get rid of
this performance problem.  This has caused 8h delays in generic/333 and
generic/334.

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