Btrfs: add more maintainers

I'm lucky to have a huge amount of help on Btrfs, and want to thank
everyone that sends patches, does review and helps track down bugs.

Dave Sterba is a long time reviewer and contributor, and adding him
to the maintainers file reflects the excellent work he has been
doing for years.

Signed-off-by: Chris Mason <clm@fb.com>

diff --cc MAINTAINERS
Simple merge