MAINTAINERS: update btrfs entry

This is an update to reflect reality, not a signal of any seismic
change. Dave Sterba has been the acting maintainer for almost a decade,
I've simply been here as a backstop in case he gets hit by a bus. The
fact is we have a strong and thriving community with any number of more
active developers that can take on that role if it's necessary. I'm
exceedingly happy and proud of the work that Dave has done in keeping us
all in line, and know that if further changes need to be made it'll be
with the development community we've built throughout the lifetime of
btrfs.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>