xfs: fix finding a last resort AG in xfs_filestream_pick_ag

In the Linux kernel, the following vulnerability has been resolved:

xfs: fix finding a last resort AG in xfs_filestream_pick_ag

When the main loop in xfs_filestream_pick_ag fails to find a suitable

AG it tries to just pick the online AG. But the loop for that uses

args->pag as loop iterator while the later code expects pag to be

set. Fix this by reusing the max_pag case for this last resort, and

also add a check for impossible case of no AG just to make sure that

the uninitialized pag doesn't even escape in theory.