CRED: Wrap task credential accesses in 9P2000 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>
Reviewed-by: Eric Van Hensbergen <ericvh@gmail.com>
Cc: Ron Minnich <rminnich@sandia.gov>
Cc: Latchesar Ionkov <lucho@ionkov.net>
Cc: v9fs-developer@lists.sourceforge.net
Signed-off-by: James Morris <jmorris@namei.org>
diff --git a/fs/9p/fid.c b/fs/9p/fid.c
index 3031e32..a43e4ab 100644
--- a/fs/9p/fid.c
+++ b/fs/9p/fid.c
@@ -120,7 +120,7 @@
 	switch (access) {
 	case V9FS_ACCESS_SINGLE:
 	case V9FS_ACCESS_USER:
-		uid = current->fsuid;
+		uid = current_fsuid();
 		any = 0;
 		break;