fuse: avoid unnecessary spinlock bump

Move dmap free worker kicker inside the critical region, so that extra
spinlock lock/unlock could be avoided.

Suggested-by: Liu Jiang <gerry@linux.alibaba.com>
Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>