LU-16510 build: include unsafe_memcpy definition

LU-16510 build: include unsafe_memcpy definition

The original LU-16510 missed a key part of the
unsafe_memcpy code from the upstream kernel, and so we
weren't actually defining unsafe_memcpy() as intended.

Thanks to Aurelien Degremont <adegremont@nvidia.com> for
pointing this out.

Fixes: 919b93b9 ("LU-16510 build: fortified memcpy from linux 6.1")
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: Ib9e2d56ed0b3691f1ab9fcd25403fa86ac784b6d
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50573
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Aurelien Degremont <adegremont@nvidia.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>