libnv: Allow use in non-sleepable contexts

libnv: Allow use in non-sleepable contexts

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
allocations.

Reviewed by:	melifaro, oshogbo
MFC after:	1 week
Sponsored by:	Rubicon Communications, LLC ("Netgate")
Differential Revision:	https://reviews.freebsd.org/D29556