staging: lustre: Remove VLA usage

The kernel would like to have all stack VLA usage removed[1]. This switches
to a simple kasprintf() instead, and in the process fixes an off-by-one
between the allocation and the sprintf (allocation did not include NULL
byte in calculation).

[1] https://lkml.org/lkml/2018/3/7/621



Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>