xfs: standardize ondisk to incore conversion for free space btrees

xfs: standardize ondisk to incore conversion for free space btrees

Source kernel commit: 35e3b9a11740b53387e7af151768c13700f80696

Create a xfs_alloc_btrec_to_irec function to convert an ondisk record to
an incore record, and a xfs_alloc_check_irec function to detect
corruption.  Replace all the open-coded logic with calls to the new
helpers and bubble up corruption reports.

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