klog: don't unconditionally call mknod()

If /dev/kmsg already exists, it's unnecessary for klog_init()
to create it's own copy. This avoids needing to grant the mknod
permission to everyone who uses kmsg. Typically the only time
/dev/kmsg doesn't exist is before ueventd starts.

(cherrypicked from commit 4d32a486)

Bug: 21242418
Change-Id: I0c88d80feca6899fcdbc8c9f2f99448ee0a3422d