MAINTAINERS: add zswap and zbud maintainer

Add maintainer information for zswap and zbud into the MAINTAINERS file.

Signed-off-by: Seth Jennings <sjenning@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit 0cf31ec10e92253e2908cd830145a71043740d77)