xfs: clean up scrub usage of KM_NOFS

All scrub code runs in transaction context, which means that memory
allocations are automatically run in PF_MEMALLOC_NOFS context.  It's
therefore unnecessary to pass in KM_NOFS to allocation routines, so
clean them all out.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>