Recursion on the free page queue mutex occurred when UMA needed to allocate
a new page of radix trie nodes to complete a vm_radix_insert() operation
that was requested by vm_page_cache(). Specifically, vm_page_cache()
already held the free page queue lock when UMA tried to acquire it through
a call to vm_page_alloc(). This code path no longer exists, so there is no
longer any reason to allow recursion on the free page queue mutex.

Improve nearby comments.

Reviewed by: kib, markj
Tested by: pho
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D8628