ksmbd: use F_SETLK when unlocking a file

ksmbd seems to be trying to use a cmd value of 0 when unlocking a file.
That activity requires a type of F_UNLCK with a cmd of F_SETLK. For
local POSIX locking, it doesn't matter much since vfs_lock_file ignores
@cmd, but filesystems that define their own ->lock operation expect to
see it set sanely.

Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: David Howells <dhowells@redhat.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>