rtnet: tcp: Remove duplicate copy_from/to_user

The RTNET sendmsg/recvmsg protocol handlers used to call
copy_to/from_user on the struct user_msghdr argument. The syscall entry
code already does this copy, so calling again the copy_to/from_user in
handlers triggers SPECTRE mitigation protection on ARM. This patch
removes the calls in the handlers.

This patch has not been tested

Signed-off-by: François LEGAL <devel@thom.fr.eu.org>
[Jan: massage commit log]
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
12 jobs for stable/v3.1.x in 49 minutes and 39 seconds (queued for 8 seconds)
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