xfs: don't treat unknown di_flags2 as corruption in scrub

xchk_inode_flags2() currently treats any di_flags2 values that the
running kernel doesn't recognize as corruption, and calls
xchk_ino_set_corrupt() if they are set.  However, it's entirely possible
that these flags were set in some newer kernel and are quite valid,
but ignored in this kernel.

(Validators don't care one bit about unknown di_flags2.)

Call xchk_ino_set_warning instead, because this may or may not actually
indicate a problem.

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