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.

Reported-by:  <syzbot+4125a3c514e3436a02e6@syzkaller.appspotmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by:  <syzbot+4125a3c514e3436a02e6@syzkaller.appspotmail.com>
Fixes: f8f1ed1a ("xfs: return a referenced perag from filestreams allocator")
Cc: <stable@vger.kernel.org> # v6.3
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>