zcache: Make the debug code use pr_debug

as if you are debugging this driver you would be using 'debug'
on the command line anyhow - and this would dump the debug
data on the proper loglevel.

While at it also remove the unconditional #define ZCACHE_DEBUG.

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>