io_uring: Introduce getsockname io_uring cmd

io_uring: Introduce getsockname io_uring cmd

Introduce a socket-specific io_uring_cmd to support
getsockname/getpeername via io_uring.  I made this an io_uring_cmd
instead of a new operation to avoid polluting the command namespace with
what is exclusively a socket operation.  In addition, since we don't
need to conform to existing interfaces, this merges the
getsockname/getpeername in a single operation, since the implementation
is pretty much the same.

This has been frequently requested, for instance at [1] and more
recently in the project Discord channel. The main use-case is to support
fixed socket file descriptors.

[1] https://github.com/axboe/liburing/issues/1356

Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>