This is a step towards generic support for kernel dump compression.
dump_start() and dump_finish() are responsible for writing kernel dump
headers, optionally writing the crypto key, and initializing "dumplo",
i.e. the offset into the dump device at which we start writing the
Also remove the unused dump_write_pad(), and make some functions
static now that they're no longer used outside of kern_shutdown.c.
No functional change intended.