- merge uvm_kmguard.{c,h} into subr_kmem.c. It is only user there, and
makes it more consistent. Also, it allows us to enable KMEM_GUARD
without enabling DEBUG.
- rename uvm_kmguard_XXX to kmem_guard_XXX, for consistency
- improve kmem_guard_alloc() so that it supports allocations bigger than
PAGE_SIZE
- remove the canary value, and use directly the kmem header as underflow
pattern.
- fix some comments
(The UAF fifo is disabled for the moment; we actually need to register
the va and its size, and add a weight support not to consume too much
memory.)