fstests/MAINTAINERS: add a co-maintainer for btrfs testing part

fstests/MAINTAINERS: add a co-maintainer for btrfs testing part

Darrick J. Wong would like to nominate Anand Jain to help more on
btrfs testing part (tests/btrfs and common/btrfs). He would like to
be a co-maintainer of btrfs part, will help to review and test
fstests btrfs related patches, and I might merge from him if there's
big patchset. So CC him besides send to fstests@ list, when you have
a btrfs fstests patch.

Acked-by: David Sterba <dsterba@suse.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>