zcache: Provide accessory functions for counter decrease.

This way we can have all wrapped with these functions and
can disable/enable this with CONFIG_DEBUG_FS.

Acked-by: Dan Magenheimer <dan.magenheimer@oracle.com>
[v2: Rebase on top of 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>