zlogger: map only the actively used buffer to the userspace

zlogger: map only the actively used buffer to the userspace

Each zlogger client maps only a single 4KB buffer and writes to it. Once
it is filled, it calls ALLOC ioctl (like before) and kernel remaps a new
buffer in place of the old buffer.

If kernel needs to free a buffer, the userspace mapping is cleared and
will cause a page fault on the next access, which in turn will cause
allocation of the new buffer just like the ALLOC ioctl does.

Change-Id: I2d6fe8406e201ef20b6378a7ba37ed5df7790406
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>