fuse: no fc->lock in request_end()

fuse: no fc->lock in request_end()

No longer need to call request_end() with the connection lock held.  We
still protect the background counters and queue with fc->lock, so acquire
it if necessary.

Signed-off-by: Miklos Szeredi <[email protected]>
Reviewed-by: Ashish Samant <[email protected]>