CRED: Wrap task credential accesses in the capabilities code

CRED: Wrap task credential accesses in the capabilities code

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 <[email protected]>
Reviewed-by: James Morris <[email protected]>
Acked-by: Serge Hallyn <[email protected]>
Cc: Andrew G. Morgan <[email protected]>
Signed-off-by: James Morris <[email protected]>