[PATCH] Keys: Replace duplicate non-updateable keys rather than failing

Cause an attempt to add a duplicate non-updateable key (such as a keyring) to
a keyring to discard the extant copy in favour of the new one rather than
failing with EEXIST:

	# do the test in an empty session
	keyctl session
	# create a new keyring called "a" and attach to session
	keyctl newring a @s
	# create another new keyring called "a" and attach to session,
	# displacing the keyring added by the second command:
	keyctl newring a @s

Without this patch, the third command will fail.

For updateable keys (such as those of "user" type), the update method will
still be called rather than a new key being created.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>