xfs: ignore agfl read errors when not scrubbing agfl

In xfs_scrub_ag_read_headers, if we're not scrubbing the AGFL but
hit a read error reading the AGFL, we should reset the error code
so that it doesn't propagate up into the caller.

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