io_uring/net: isolate msghdr copying code

io_uring/net: isolate msghdr copying code

The user access section in io_msg_copy_hdr() is overextended by covering
selected buffers. It's hard to work with and prone to errors. Limit the
section to msghdr import only, selected buffers will do a separate
copy_from_user() call, and then move it into its own function. This
should be fine, selected buffer single shots are not important, for
multishots the overhead should be non-existent, and it's not that
expensive overall.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/d3eb1f81c8cfbea9f1aa57dab90c472d2aa6e371.1740569495.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>