linuxkpi: Remove extraneous NULL check on M_WAITOK allocation

The check was not introduced in r342628, but the subsequent unchecked access to
refs was added then, prompting a Coverity warning about "Null pointer
dereferences (FORWARD_NULL)." The warning is bogus due to M_WAITOK, but so is
the NULL check that hints it, so just remove it.

CID: 1398588
Reported by: Coverity