update JFS entry in MAINTAINERS

JFS hasn't really been supported for a while.  It's still maintained,
but saying it's supported is a stretch.  Updating my preferred email
address as well.

Signed-off-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>