LU-16117 build: Avoid excessive modpost warnings

LU-16117 build: Avoid excessive modpost warnings

To avoid modpost warnings about duplicate symbols do not add
the LINUX_OBJ kernel symbols to the KBUILD_EXTRA_SYMBOLS list

Lustre-change: https://review.whamcloud.com/48362
Lustre-commit: 445f85de2baddbaa8a84a709b98a9d0b8ed4231a

Test-Parameters: trivial
HPE-bug-id: LUS-11192
Signed-off-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Change-Id: I85fc90661efcb66e4aa39c9bd3393dbe4f7ba5eb
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Petros Koutoupis <petros.koutoupis@hpe.com>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/49849
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>