ksmbd: add channel rwlock

[ Upstream commit 8e06b31e ]

Add missing rwlock for channel list in session.

Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Reviewed-by: Hyunchul Lee <hyc.lee@gmail.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Stable-dep-of: 7b432337 ("ksmbd: fix deadlock in ksmbd_find_crypto_ctx()")
Signed-off-by: Sasha Levin <sashal@kernel.org>