xfs: decode scrub flags in ftrace output

When using pretty-printed scrub tracepoints, decode the meaning of the
scrub flags as strings for easier reading.

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