zbud: Fix compile warnings due to usage of debugfs_create_size_t

.
drivers/staging/zcache/zbud.c:336: warning: passing argument 4 of ‘debugfs_create_size_t’ from incompatible pointer type
include/linux/debugfs.h:80: note: expected ‘size_t *’ but argument is of type ‘long unsigned int *’
..

which is b/c we end up using 'unsigned' or 'unsigned long' instead
of 'ssize_t'. So lets fix this up and use the proper type.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>