MAINTAINERS: update entry for fs/locks.c

Both Bruce and I have done a fair bit of work in these files recently,
and would like to be notified if anyone is proposing changes to it.

Also, Matthew is no longer interested in maintaining this code, so
remove him.

Cc: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Acked-by: "J. Bruce Fields" <bfields@fieldses.org>