xfs: stringify scrub types in ftrace output

Source kernel commit: 86d163dbfe2ac0b30fbb6e256301abbfa9e4549e

Use __print_symbolic to print the scrub type in ftrace output.

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