io_uring/net: unify *mshot_prep calls with compat

io_uring/net: unify *mshot_prep calls with compat

Instead of duplicating a io_recvmsg_mshot_prep() call in the compat
path, let the common code handle it. For that, copy necessary compat
fields into struct user_msghdr. Note, it zeroes user_msghdr to be on the
safe side as compat is not that interesting and overhead shouldn't be
high.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/94e62386dec570f83b4a4270a46ac60bc415fb71.1740569495.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>