zio-buf-kmalloc: add nitem_stored

nitem tell how many block item was allocated, but the user is interested in
the block actually stored in the buffer.

The in FMC ADC is much evident the problem where the driver can allocate
more than one block for each arm(). For example, the driver can allocate
10 blocks but the stored blocks are 0 until all acquisition complete.

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
zio-buf-kmalloc: add nkb_stored

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>