staging: lustre: discard libcfs_kvzalloc_cpt()

This function is used precisely once, and is sufficiently
trivial that it may as well be open-coded.

Doing so helpfully highlights the similarity
between the new kvzalloc_node() call and the already existing
kzalloc_node() call in the same function.

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