zio-buf-vmalloc: new buffer type

This buffer uses vmalloc and allows mmap from user space.  It started
as a copy of buf-kmalloc, so you can diff to change what is needed
to support mmap over a read/write buffer type.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Acked-by: Federico Vaga <federico.vaga@gmail.com>