44c125c4cebc2fd87c6260b90eddae11201f5232 switched the nvlist allocations
to be M_WAITOK, but this precludes the use in non-sleepable contexts.
(E.g. with a nonsleepable lock held).
All callers for these allocation functions already cope with memory
alloation failures, so there's no reason to allow sleeping during
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")