KEYS: Implement a big key type that can save to tmpfs

Implement a big key type that can save its contents to tmpfs and thus
swapspace when memory is tight.  This is useful for Kerberos ticket caches.

Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Simo Sorce <simo@redhat.com>

diff --cc include/keys/big_key-type.h
Simple merge

diff --cc include/linux/key.h
Simple merge

diff --cc security/keys/Kconfig
Simple merge

diff --cc security/keys/Makefile
Simple merge

diff --cc security/keys/big_key.c
Simple merge