libata: change maintainer

Jeff is leaving for something more interesting and I'm inheriting the
maintainership of libata.  Thanks a lot for the good work and have
fun, Jeff!

v2: The original path forgot to update git tree URL.  Updated.
    Spotted by Sergei.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jeff Garzik <jgarzik@pobox.com>
Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>