CIFS: Check for mandatory brlocks on read/write

Currently CIFS code accept read/write ops on mandatory locked area
when two processes use the same file descriptor - it's wrong.
Fix this by serializing io and brlock operations on the inode.

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