zcache: Fix compile warnings due to usage of debugfs_create_size_t

When we compile we get tons of:
include/linux/debugfs.h:80:16: note: expected ‘size_t *’ but argument is
of type ‘long int *’
drivers/staging/zcache/zcache-main.c:279:2: warning: passing argument 4
of ‘debugfs_create_size_t’ from incompatible pointer type [enabled by d
efault]

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.

[v2: Rebased directly on staging]
Acked-by: Dan Magenheimer <dan.magenheimer@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>