staging: android: ram_console: honor dmesg_restrict

The Linux kernel has a setting called dmesg_restrict. When true,
only processes with CAP_SYSLOG can view the kernel dmesg logs. This
helps prevent leaking of kernel information into user space.

On Android, it's possible to bypass these restrictions by viewing
/proc/last_kmsg.

This change makes /proc/last_kmsg require the same permissions as
dmesg.

CC: Android Kernel Team <kernel-team@android.com>
Signed-off-by: Nick Kralevich <nnk@google.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>