cma: fix race condition on a page

cruel, brute-force method for letting cma/migration to
finish its job without stealing the lock
migration_entry_wait() and creating a live-lock on the
faulted page. This patch solves the case of
page->_count == 2 migration failure.

Change-Id: Ia94542a80e44a213831291af289bbf5ee6880bfd
Signed-off-by: Heesub Shin <heesub.shin@samsung.com>
Reviewed-on: http://165.213.202.130:8080/39341


Tested-by: System S/W SCM <scm.systemsw@samsung.com>
Tested-by: Dongjun Shin <d.j.shin@samsung.com>
Reviewed-by: Hyunju Ahn <hyunju.ahn@samsung.com>
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>