ksmbd: prevent connection release during oplock break notification

ksmbd_work could be freed when after connection release.
Increment r_count of ksmbd_conn to indicate that requests
are not finished yet and to not release the connection.

Cc: stable@vger.kernel.org
Reported-by: Norbert Szetei <norbert@doyensec.com>
Tested-by: Norbert Szetei <norbert@doyensec.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>