staging: lustre: 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.

Also remove a nearby comment that doesn't relate to the
(remaining) code at all.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>