CRED: Wrap task credential accesses in the filesystem subsystem

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: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: James Morris <jmorris@namei.org>
diff --git a/fs/namespace.c b/fs/namespace.c
index cce4670..d8bc2c4 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1176,7 +1176,7 @@
 	if (S_ISLNK(path->dentry->d_inode->i_mode))
 		return -EPERM;
 	if (path->dentry->d_inode->i_mode & S_ISVTX) {
-		if (current->uid != path->dentry->d_inode->i_uid)
+		if (current_uid() != path->dentry->d_inode->i_uid)
 			return -EPERM;
 	}
 	if (inode_permission(path->dentry->d_inode, MAY_WRITE))