xfs: fix incorrect unit conversion in scrub tracepoint

XFS_DADDR_TO_FSB converts a raw disk address (in units of 512b blocks)
to a raw disk address (in units of fs blocks).  Unfortunately, the
xchk_block_error_class tracepoints incorrectly uses this to decode
xfs_daddr_t into segmented AG number and AG block addresses.  Use the
correct translation code.

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