mm/page_owner: ask users about default setting of PAGE_OWNER

Since this commit 48c96a36 ("mm/page_owner: keep track
of page owners") doesn't enable the page_owner by default
even though CONFIG_PAGE_OWNER is enabled.

Add configuration option CONFIG_PAGE_OWNER_ENABLE_DEFAULT to
allow user to enable it by default through the defconfig file.

CRs-Fixed: 1006743
Change-Id: I9b565a34e2068bf575974eaf3dc9f7820bdd7a96
Signed-off-by: Trilok Soni <tsoni@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>
[swatsrid@codeaurora.org: Fix merge errors and warnings]
Signed-off-by: Swathi Sridhar <swatsrid@codeaurora.org>