block/io_uring: improve error message when init fails

block/io_uring: improve error message when init fails

The man page for io_uring_queue_init states:

> io_uring_queue_init(3) returns 0 on success and -errno on failure.

and the man page for io_uring_setup (which is one of the functions
where the return value of io_uring_queue_init() can come from) states:

> On error, a negative error code is returned. The caller should not
> rely on errno variable.

Tested using 'sysctl kernel.io_uring_disabled=2'. Output before this
change:

> failed to init linux io_uring ring

Output after this change:

> failed to init linux io_uring ring: Operation not permitted

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20240123135044.204985-1-f.ebner@proxmox.com>