CRED: Wrap task credential accesses in the UBIFS filesystem

Wrap access to task credentials so that they can be separated more easily from
the task_struct during the introduction of COW creds.

Change most current->(|e|s|fs)[ug]id to current_(|e|s|fs)[ug]id().

Change some task->e?[ug]id to task_e?[ug]id().  In some places it makes more
sense to use RCU directly rather than a convenient wrapper; these will be
addressed by later patches.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: James Morris <jmorris@namei.org>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Cc: Artem Bityutskiy <dedekind@infradead.org>
Cc: Adrian Hunter <ext-adrian.hunter@nokia.com>
Cc: linux-mtd@lists.infradead.org
Signed-off-by: James Morris <jmorris@namei.org>