[PATCH] doc: refer to kdump in oops-tracing.txt

Kdump has been merged and supported on several architectures.  It is better
to encourage to use kdump rather than non standard kernel crash dump
patches.

Signed-off-by: Akinobu Mita <mita@miraclelinux.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>