cifs: fix stray unlock in cifs_chan_skip_or_disable

A recent change moved the code that decides to skip
a channel or disable multichannel entirely, into a
helper function.

During this, a mutex_unlock of the session_mutex
should have been removed. Doing that here.

Fixes: f591062b

 ("cifs: handle servers that still advertise multichannel after disabling")
Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>