fuse: add fuse_dio_lock/unlock helper functions

So far this is just a helper to remove complex locking logic out of
fuse_direct_write_iter.  Especially needed by the next patch in the series
to that adds the fuse inode cache IO mode and adds in even more locking
complexity.

Signed-off-by: Bernd Schubert <bschubert@ddn.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>