Add an mbuf allocator for netdump.

The aim is to permit mbuf allocations after a panic without calling into
the page allocator, without imposing any runtime overhead during regular
operation of the system, and without modifying driver code. The approach
taken is to preallocate a number of mbufs and clusters, storing them
in linked lists, and using the lists to back some UMA cache zones. At
panic time, the mbuf and cluster zone pointers are overwritten with
those of the cache zones so that the mbuf allocator returns
preallocated items.

Using this scheme, drivers which cache mbuf zone pointers from
m_getzone() require special handling when implementing netdump support.

Reviewed by: cem (earlier version), julian, sbruno
MFC after: 1 month
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D15251