xfs_repair: stop using ->data_entry_p()

xfs_repair: stop using ->data_entry_p()

The ->data_entry_p() op went away in v5.5 kernelspace, so rework
xfs_repair to use ->data_entry_offset instead, in preparation
for the v5.5 libxfs backport.

This could later be cleaned up to use offsets as was done
in kernel commit 8073af5153c for example.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>