Revert "zram: fix error return code"

This change is unneeded since ret was already initialized at the top of
the function.  While harmless, revert it so our zram backport is
identical to 3.18 (modulo the block layer changes).

This reverts commit fc6ca6bd.

Bug: 25951511

Change-Id: Ia43d14b5417aa6c0b89a4b50a62075a1a669181c
Signed-off-by: Greg Hackmann <ghackmann@google.com>