mptcp: use copy_from_iter helpers on transmit

mptcp: use copy_from_iter helpers on transmit

The perf traces show an high cost for the MPTCP transmit path memcpy.

It turn out that the helper currently in use carries quite a bit
of unneeded overhead, e.g. to map/unmap the memory pages.

Moving to the 'copy_from_iter' variant removes such overhead and
additionally gains the no-cache support.

Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20231023-send-net-next-20231023-2-v1-6-9dc60939d371@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>