x86: get rid of small constant size cases in raw_copy_{to,from}_user()

[ Upstream commit 4b842e4e ]

Very few call sites where that would be triggered remain, and none
of those is anywhere near hot enough to bother.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Sasha Levin <sashal@kernel.org>