zio-buf-kmalloc.c: use curlen and curkb sysfs attribute

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
FIXME sysfs.c: flush: clear also user_block

Flushing after reading the last block corrupts nitem counter because the
user_block is still there until next read (write)

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>