fuse: Fix the condition to enable over-io-uring

The existing condition in fuse_uring_cmd() is there only to avoid
disabling io-uring for connections that already run with it, missing
was a condition to refuse any IORING_OP_URING_CMD if the
connection/channel didn't get enabled because of missing FUSE_INIT
reply flag FUSE_OVER_IO_URING. Without the reply flag the barrier in
fuse_uring_ready() doesn't work and IO could already be going on and
cause deadlock states (at a minimum one between fch->bg_lock and
queue->lock).

The change itself is trivial, but brings behavior change,
FUSE_OVER_IO_URING has to be set in the FUSE_INIT_REPLY by fuse servers
to accept any IORING_OP_URING_CMD. Libfuse does that and the only
non-libfuse implementation I found (fractal-fuse) also does it.
Qemu patches for fuse-io-uring are not merged yet, as far as I know.

Moved up is the smp_load_acquire(&fch->initialized) check, as a
fuse-server implementation might try to setup io-uring before FUSE_INIT
is processed and might have gotten -EOPNOTSUPP instead of -EAGAIN.

Also fixed is a stale comment that explains the handling of the
FUSE_OVER_IO_URING flag in early RFC versions.

If there should be a report from any library or application we
probably need to revert this commit.

Fixes: 3393ff964e0f ("fuse: block request allocation until io-uring init is complete")
Signed-off-by: Bernd Schubert <bernd@bsbernd.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>