Previously, vm_radix_remove() would panic if the radix trie didn't

Previously, vm_radix_remove() would panic if the radix trie didn't
contain a vm_page_t at the specified index.  However, with this
change, vm_radix_remove() no longer panics.  Instead, it returns NULL
if there is no vm_page_t at the specified index.  Otherwise, it
returns the vm_page_t.  The motivation for this change is that it
simplifies the use of radix tries in the amd64, arm64, and i386 pmap
implementations.  Instead of performing a lookup before every remove,
the pmap can simply perform the remove.

Reviewed by:	kib, markj
Differential Revision:	https://reviews.freebsd.org/D8708