MAINTAINERS: fscrypt: update mailing list, patchwork, and git

Now that there has been a dedicated mailing list, patchwork project, and
git repository set up for filesystem encryption, update the MAINTAINERS
file accordingly.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>