fuse: don't let fuse_req->end() put the base reference

fuse_req->end() was supposed to be put the base reference but there's
no reason why it should.  It only makes things more complex.  Move it
out of ->end() and make it the responsibility of request_end().

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>