staging: android: persistent_ram: make persistent_ram_write atomic

Allow persistent_ram_write to be called on multiple cpus at
the same time, as long as ecc is not in use.  Uses atomics
for the buffer->start and buffer->size counters.

[jstultz: Fix up some pr_info casting issues on 64bit]
CC: Greg KH <gregkh@linuxfoundation.org>
CC: Android Kernel Team <kernel-team@android.com>
Signed-off-by: Colin Cross <ccross@android.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Change-Id: If8ddd7913ed92423556207ccbbb5e3c67d038d0b