virtio_fs: Remove redundant spinlock in virtio_fs_request_complete()

Since clear_bit is an atomic operation, the spinlock is redundant and
can be removed, reducing lock contention is good for performance.

Signed-off-by: Li RongQing <lirongqing@baidu.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>