LU-9859 libcfs: discard MKSTR() macro

LU-9859 libcfs: discard MKSTR() macro

This is only used for tracing when some strings might
be NULL.  NULL strings are not a problem for tracing,
vnsprintf() will report them as "(null)" which is probably
better (easier to parse) than an empty string.

Linux-commit: dd0393a5f29633f0e3d52e4c26ae4123c873c016

Test-Parameters: trivial
Change-Id: Ia305ea0d0dc05602a03dea589f928b6a599ee55e
Signed-off-by: Mr. NeilBrown <neilb@suse.de>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50376
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>