CIFS: Turn lock mutex into rw semaphore

and allow several processes to walk through the lock list and read
can_cache_brlcks value if they are not going to modify them.

Signed-off-by: Pavel Shilovsky <pshilovsky@etersoft.ru>
Signed-off-by: Steve French <sfrench@us.ibm.com>

diff --cc fs/cifs/cifsfs.c
Simple merge

diff --cc fs/cifs/cifsglob.h
Simple merge

diff --cc fs/cifs/file.c
Simple merge

diff --cc fs/cifs/smb2file.c
Simple merge