init: make initrd/initramfs decompression failure a KERN_EMERG event

Impact: More consistent behaviour, avoid policy in the kernel

Upgrade/downgrade initrd/initramfs decompression failure from
inconsistently a panic or a KERN_ALERT message to a KERN_EMERG event.
It is, however, possible do design a system which can recover from
this (using the kernel builtin code and/or the internal initramfs),
which means this is policy, not a technical necessity.

A good way to handle this would be to have a panic-level=X option, to
force a panic on a printk above a certain level.  That is a separate
patch, however.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>