zcache: Move debugfs code out of zcache-main.c file.

Note that at this point there is no CONFIG_ZCACHE_DEBUG
option in the Kconfig. So in effect all of the counters
are nop until that option gets re-introduced in:
zcache/debug: Coalesce all debug under CONFIG_ZCACHE_DEBUG

Acked-by: Dan Magenheimer <dan.magenheimer@oracle.com>
[v1: Fixed conflicts due to rebase]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>