make buffer size match kernel max transfer size

make buffer size match kernel max transfer size

Currently libfuse has a hardcoded buffer limit to 128kib, while fuse
kernel module has a limit up to 32 pages.

This patch changes buffer limit to match the current page size, instead
of assuming 4096 bytes pages, enabling architectures with bigger pages
to use larger buffers, improving performance.

Also, add a new macro (HEADER_SIZE) to specify the space needed to
accommodate the header, making it easier to understand why those extra
4096 bytes are needed

Signed-off-by: Carlos Maiolino <cmaiolino-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>