CRED: Missing put_cred() in prepare_kernel_cred()

Missing put_cred() in the error handling path of prepare_kernel_cred().

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Steve Dickson <steved@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>