Cleanup the LinuxKPI slab implementation.

Put large functions into linux_slab.c instead of declaring them static
inline.

Add support for more memory allocation wrappers like kmalloc_array()
and __vmalloc().

Make sure either the M_WAITOK or the M_NOWAIT flag is set and mask
away unused memory allocation flags before calling FreeBSD's malloc()
routine.

Move kmalloc_node() definition to slab.h where it belongs.

Implement support for the SLAB_DESTROY_BY_RCU feature when creating a
kmem_cache which basically means kmem_cache memory is freed using
call_rcu().

MFC after: 1 week
Sponsored by: Mellanox Technologies