MAINTAINERS: add ptrace entry

Add Roland and Oleg as formal ptrace maintainers, they've been doing the
job for a while.

Includes the file patterns requested by Joe.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Roland McGrath <roland@redhat.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>