drivers/block/zram/zram_drv.c: fix error return codes not being returned in writeback_store

Currently when an error code -EIO or -ENOSPC in the for-loop of
writeback_store the error code is being overwritten by a ret = len
assignment at the end of the function and the error codes are being
lost.  Fix this by assigning ret = len at the start of the function and
remove the assignment from the end, hence allowing ret to be preserved
when error codes are assigned to it.

Addresses Coverity ("Unused value")

Link: http://lkml.kernel.org/r/20191128122958.178290-1-colin.king@canonical.com
Fixes: a939888e

 ("zram: support idle/huge page writeback")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>