smb3: fix lock breakage for cached writes

Mandatory locking is enforced for cached writes, which violates
default posix semantics, and also it is enforced inconsistently.
This apparently breaks recent versions of libreoffice, but can
also be demonstrated by opening a file twice from the same
client, locking it from handle one and writing to it from
handle two (which fails, returning EACCES).

Since there was already a mount option "forcemandatorylock"
(which defaults to off), with this change only when the user
intentionally specifies "forcemandatorylock" on mount will we
break posix semantics on write to a locked range (ie we will
only fail the write in this case, if the user mounts with
"forcemandatorylock").

Fixes: 85160e03 ("CIFS: Implement caching mechanism for mandatory brlocks")
Cc: stable@vger.kernel.org
Cc: Pavel Shilovsky <piastryyy@gmail.com>
Reported-by:  <abartlet@samba.org>
Reported-by: Kevin Ottens <kevin.ottens@enioka.com>
Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>