xfs: remove xfs_filestream_select_ag() longest extent check

Picking a new AG checks the longest free extent in the AG is valid,
so there's no need to repeat the check in
xfs_filestream_select_ag(). Remove it.

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