Remove all the fnvlist*nosleep functions.  The ability to allocate
the nvlists with nvlist_alloc() and KM_PUSHPAGE will cause subsequent
operations to use the appropriate handler.  Convert the allocations
of the various SPA history nvlists to use VERIFY0(nvlist_alloc(&nvl,
NV_UNIQUE_NAME, KM_PUSHPAGE)).
KM_PUSHPAGE-ize the fnvlist_pack and fnvlist_dup in the SPA history path.