CRED: Wrap task credential accesses in the Ext4 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: Stephen Tweedie <sct@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: adilger@sun.com
Cc: linux-ext4@vger.kernel.org
Signed-off-by: James Morris <jmorris@namei.org>

fs/ext4/balloc.c

@@ -624,7 +624,7 @@ int ext4_has_free_blocks(struct ext4_sb_info *sbi, s64 nblocks)
 		return 1;
 
 	/* Hm, nope.  Are (enough) root reserved blocks available? */
-	if (sbi->s_resuid == current->fsuid ||
+	if (sbi->s_resuid == current_fsuid() ||
 	    ((sbi->s_resgid != 0) && in_group_p(sbi->s_resgid)) ||
 	    capable(CAP_SYS_RESOURCE)) {
 		if (free_blocks >= (nblocks + dirty_blocks))