UPSTREAM: list: Split list_del() debug checking into separate function

(cherry-picked from 0cd340dc)

Similar to the list_add() debug consolidation, this commit consolidates
the debug checking performed during CONFIG_DEBUG_LIST into a new
__list_del_entry_valid() function, and stops list updates when corruption
is found.

Refactored from same hardening in PaX and Grsecurity.

Change-Id: I9e3b8654ab25f3a196e3336fc4882b73010873e7
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Satya Tangirala <satyat@google.com>
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>