xfs: use GPF_NOFS when allocating btree cursors

Use NOFS for allocating btree cursors, since they can be called
under the ilock.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>