zio-buf-kmalloc: fail on alloc, don't fail on store

This is a fix for a design bug for this buffer. Actually, the
vmalloc buffer already is like this (and cannot be different).
This allows simplifying chardev.c and fix some corner cases that
are currently misbehaving.

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