vm: stop passing M_ZERO when allocating radix nodes

Allocation explicitely initialized the 3 leading fields. The rest is an
array which is supposed to be NULL-ed prior to deallocation.

Delegate zeroing to the infrequently called object initializator.

This gets rid of one of the most common memset consumers.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D15989