Previous kernels have allowed opening the CPU frequency file regardless
whether the CPU is up or not. This fixes some log spam on one device
with dynamic hot plug CPU feature, which does not allow opening the CPU
frequency file if CPU is down.
Also, since the file descriptors are global and have long lives, add
the close-on-exec flag.
Change-Id: Ia14a2b9e20038dfb96a573920176a47a96bd3f5a
542af12d10