LU-9859 libcfs: move kernel specific code out of libcfs core

LU-9859 libcfs: move kernel specific code out of libcfs core

Over time kernel version specific code has leaked into the libcfs
core code. Move that code to the linux subdirectory code so in
the future code cleanup is not missed.

Lustre-change: https://review.whamcloud.com/52010
Lustre-commit: 8754693fe6ddac4b74e27800a05d5aea00bb0359

Test-Parameters: trivial
Change-Id: I38a00c377334066160083edd3932d4a718198497
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56763
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>