MAINTAINERS: Fix remaining pointers to obsolete libata.git

libata.git no longer exists.  Replace the remaining pointers to it by
pointers to the block tree, which is where all libata development
happens now.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Jens Axboe <axboe@kernel.dk>