Staging: lustre: lnet: Remove explicit NULL comparision

Replaced explicit NULL comparision with its simplier form.
Found using coccinelle:

@replace_rule@
expression e;
@@

-e == NULL
+ !e

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>