fuse: get rid of ff->readdir.lock

The same protection is provided by file->f_pos_lock.

Note, this relies on the fact that file->f_mode has FMODE_ATOMIC_POS.
This flag is cleared by stream_open(), which would prevent locking of
f_pos_lock.

Prior to commit 7de64d52 ("fuse: break up fuse_open_common()")
FOPEN_STREAM on a directory would cause stream_open() to be called.
After this commit this is not done anymore, so f_pos_lock will always
be locked.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>