zcache: Provide accessory functions for counter increase

This is the first step in moving the debugfs code out of the
main file in-to another file. And also allow the code to run
without CONFIG_DEBUG_FS defined.

Acked-by: Dan Magenheimer <dan.magenheimer@oracle.com>
[v2: Rebase on top staging/zcache: Fix/improve zcache writeback code, tie to a config option]
[v3: Rebase on top of zcache: Fix compile warnings due to usage of debugfs_create_size_t]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>