[PATCH] uml: update Kconfig help

The MODE_TT help was a little outdated.  This updates it in light of the
existence of skas0 mode.  It's also turned off by default since it is mostly
obsoleted by skas0 mode.

Signed-off-by: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>