kernel - Optimize the x86-64 lwbuf API

kernel - Optimize the x86-64 lwbuf API

* Change lwbuf_alloc(m) to lwbuf_alloc(m, &lwb_cache), passing a pointer to
  a struct lwb which lwbuf_alloc() may used if it desires.

* The x86-64 lwbuf_alloc() now just fills in the passed lwb and returns it.
  The i386 lwbuf_alloc() still uses the objcache w/ its kva mappings.  This
  removes objcache calls from the critical path.

* The x86-64 lwbuf_alloc()/lwbuf_free() functions are now inlines (ALL x86-64
  lwbuf functions are now inlines).