xfs: use memset to initialize xfs_scrub_agfl_info

Apparently different gcc versions have competing and
incompatible notions of how to initialize at declaration,
so just give up and fall back to the time-tested memset().

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