io_uring/net: improve the usercopy for sendmsg/recvmsg

We're spending a considerable amount of the sendmsg/recvmsg time just
copying in the message header. And for provided buffers, the known
single entry iovec.

Be a bit smarter about it and enable/disable user access around our
copying. In a test case that does both sendmsg and recvmsg, the
runtime before this change (averaged over multiple runs, very stable
times however):

Kernel		Time		Diff
====================================
-git		4720 usec
-git+commit	4311 usec	-8.7%

and looking at a profile diff, we see the following:

0.25%     +9.33%  [kernel.kallsyms]     [k] _copy_from_user
4.47%     -3.32%  [kernel.kallsyms]     [k] __io_msg_copy_hdr.constprop.0

where we drop more than 9% of _copy_from_user() time, and consequently
add time to __io_msg_copy_hdr() where the copies are now attributed to,
but with a net win of 6%.

In comparison, the same test case with send/recv runs in 3745 usec, which
is (expectedly) still quite a bit faster. But at least sendmsg/recvmsg is
now only ~13% slower, where it was ~21% slower before.

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