fs: add Jan Kara as reviewer

Jan's been really essential in help deal with reviews in a bunch of
areas and we should really make him an official reviewer. This is long
overdue imho.

Link: https://lore.kernel.org/r/20231205-aufkam-neukunden-d14970a0a6cc@brauner
Signed-off-by: Christian Brauner <brauner@kernel.org>