xfs: fix a few erroneous process_error calls in the scrubbers

There are a few places where we make a libxfs api call on behalf of some
object other than the one we're scrubbing but inadvertently call the
regular process_error function.  When this happens we mark the object
corrupt even though it was corruption in /some other/ object that
actually produced the -EFSCORRUPTED code.  The correct output flag for
these situations is SCRUB_OFLAG_XFAIL, not SCRUB_OFLAG_CORRUPT, so fix
this now that we also have a helper to set these.

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