block: only read from sqe on initial invocation of blkdev_uring_cmd()

block: only read from sqe on initial invocation of blkdev_uring_cmd()

This passthrough helper currently only supports discards. Part of that
command is the start and length, which is read from the SQE. It does
so on every invocation, where it really should just make it stable
on the first invocation. This avoids needing to copy the SQE upfront,
as we only really need those two 8b values stored in our per-req
payload.

Cc: stable@vger.kernel.org # 6.17+
Signed-off-by: Jens Axboe <axboe@kernel.dk>