zio: add current len and kb sysfs buf attribute

This patch add two new attributes: "max-buffer-len" and "max-buffer-kb".

The purpose of these attribute is to export to user space information about
the number of blocks stored in a buffer. This information can be used to
monitoring the buffer status; for example, if the buffer is full at 50%
an application can decide to read the blocks instead of start new
acquisitions

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
zio-buf-kmalloc.c: use curlen and curkb sysfs attribute

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
USELESS NOW? buffers: do not pull_block if buffer is disabled

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>