LU-18141 libcfs: don't emit a warning for LIBCFS_FREE(NULL)

LU-18141 libcfs: don't emit a warning for LIBCFS_FREE(NULL)

There is no need to emit a console message when doing
a LIBCFS_FREE() of NULL. We ought to silently accept
NULL and do nothing.

Test-Parameters: trivial
Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: If7217f4791998b15e92485a80b6f2ed051f7eecc
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56050
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: jsimmons <jsimmons@infradead.org>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>