fs/9p: fix uaf in in v9fs_stat2inode_dotl

The incorrect logical order of accessing the st object code in v9fs_fid_iget_dotl
is causing this uaf.

Fixes: 724a0845 ("fs/9p: simplify iget to remove unnecessary paths")
Reported-and-tested-by:  <syzbot+7a3d75905ea1a830dbe5@syzkaller.appspotmail.com>
Signed-off-by: Lizhi Xu <lizhi.xu@windriver.com>
Tested-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Dominique Martinet <asmadeus@codewreck.org>
Signed-off-by: Eric Van Hensbergen <ericvh@kernel.org>