KEYS: Define a __key_get() wrapper to use rather than atomic_inc()

Define a __key_get() wrapper to use rather than atomic_inc() on the key usage
count as this makes it easier to hook in refcount error debugging.

Signed-off-by: David Howells <dhowells@redhat.com>

diff --cc Documentation/security/keys.txt
Simple merge

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

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

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

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