ksmbd: fix UAF issue from opinfo->conn

If opinfo->conn is another connection and while ksmbd send oplock break
request to cient on current connection, The connection for opinfo->conn
can be disconnect and conn could be freed. When sending oplock break
request, this ksmbd_conn can be used and cause user-after-free issue.
When getting opinfo from the list, ksmbd check connection is being
released. If it is not released, Increase ->r_count to wait that connection
is freed.

Cc: stable@vger.kernel.org
Reported-by: Per Forlin <per.forlin@axis.com>
Tested-by: Per Forlin <per.forlin@axis.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>