userns: Convert security/keys to the new userns infrastructure

- Replace key_user ->user_ns equality checks with kuid_has_mapping checks.
- Use from_kuid to generate key descriptions
- Use kuid_t and kgid_t and the associated helpers instead of uid_t and gid_t
- Avoid potential problems with file descriptor passing by displaying
  keys in the user namespace of the opener of key status proc files.

Cc: linux-security-module@vger.kernel.org
Cc: keyrings@linux-nfs.org
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

init/Kconfig

@@ -927,7 +927,6 @@ config UIDGID_CONVERTED
 	# Features
 	depends on IMA = n
 	depends on EVM = n
-	depends on KEYS = n
 	depends on AUDIT = n
 	depends on AUDITSYSCALL = n
 	depends on TASKSTATS = n