msm8960: defconfig: enable slub_debug by default

This reverts commit 3aa5d76f, which
removed overhead of page poisoning as performance and power measurement
began to gain importance. Now that msm8960-perf_defconfig is in place
and is in fact the default boot img for msm8960 Android kernel it is
safe to again enable the useful memory corruption detection features
of SLUB debug. Several developers mentioned how helpful the detection
was for them.

Change-Id: I9f962a7062b9b5d190cfa9aae616202979e0215d
Signed-off-by: Jin Hong <jinh@codeaurora.org>