xfs: return EINTR when a fatal signal terminates scrub

If the program calling online fsck is terminated with a fatal signal,
bail out to userspace by returning EINTR, not EAGAIN.  EAGAIN is used by
scrubbers to indicate that we should try again with more resources
locked, and not to indicate that the operation was cancelled.  The
miswiring is mostly harmless, but it shows up in the trace data.

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