io_uring/cmd: introduce IORING_URING_CMD_REISSUE flag

Add a flag IORING_URING_CMD_REISSUE that ->uring_cmd() implementations
can use to tell whether this is the first or subsequent issue of the
uring_cmd. This will allow ->uring_cmd() implementations to store
information in the io_uring_cmd's pdu across issues.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Acked-by: David Sterba <dsterba@suse.com>
Link: https://lore.kernel.org/r/20250708202212.2851548-3-csander@purestorage.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>