debugfs: add helpers for exporting a size_t simple value

In the same spirit as debugfs_create_*(), introduce helpers for
exporting size_t values over debugfs.

The only trick done is that the format verifier is kept at %llu
instead of %zu; otherwise type warnings would pop up:

format ‘%zu’ expects type ‘size_t’, but argument 2 has type ‘long long unsigned int’

There is no real way to fix this one--however, we can consider %llu
and %zu to be compatible if we consider that we are using the same for
validating in debugfs_create_{x,u}{8,16,32}().

Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>