KEYS: Make BIG_KEYS boolean

Having the big_keys functionality as a module is very marginally useful.
The userspace code that would use this functionality will get odd error
messages from the keys layer if the module isn't loaded.  The code itself
is fairly small, so just have this as a boolean option and not a tristate.
Signed-off-by: Josh Boyer <jwboyer@fedoraproject.org>
Signed-off-by: David Howells <dhowells@redhat.com>

security/keys/Kconfig

@@ -38,7 +38,7 @@ config PERSISTENT_KEYRINGS
 	  removed if they expire (a default timeout is set upon creation).
 
 config BIG_KEYS
-	tristate "Large payload keys"
+	bool "Large payload keys"
 	depends on KEYS
 	depends on TMPFS
 	help