LU-16509 lnet: memcpy false positive in brw_test

LU-16509 lnet: memcpy false positive in brw_test

The flexible array at the end of srcp_bulk is triggering a
false positive in fortified memcpy().  Quash it with
unsafe_memcpy().

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I13386c0a8e73b04af8d398aa49361bfdf6a05ad8
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50185
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>