fuse: revalidate: don't invalidate if interrupted

If the LOOKUP request triggered from fuse_dentry_revalidate() is
interrupted, then the dentry will be invalidated, possibly resulting in
submounts being unmounted.

Reported-by: Xu Rongbo <xurongbo@baidu.com>
Closes: https://lore.kernel.org/all/CAJfpegswN_CJJ6C3RZiaK6rpFmNyWmXfaEpnQUJ42KCwNF5tWw@mail.gmail.com/


Fixes: 9e6268db ("[PATCH] FUSE - read-write operations")
Cc: <stable@vger.kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>