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>
Acked-by: Alessandro Rubini <rubini@gnudd.com>
buffers: use alloc_len and alloc_kb sysfs attribute

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