zram: add Designated Reviewer for zram in MAINTAINERS

Sergey Senozhatsky has contributed/reviewed to zram for a long time.  He
is really helpful for maintaining zram so I want for him to continue
helping me as Designated Reviewer unless he hates it.

Signed-off-by: Minchan Kim <minchan@kernel.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>