Introduce a handy list_first_entry macro

There are many places in the kernel where the construction like

   foo = list_entry(head->next, struct foo_struct, list);

are used.
The code might look more descriptive and neat if using the macro

   list_first_entry(head, type, member) \
             list_entry((head)->next, type, member)

Here is the macro itself and the examples of its usage in the generic code.
 If it will turn out to be useful, I can prepare the set of patches to
inject in into arch-specific code, drivers, networking, etc.

Signed-off-by: Pavel Emelianov <xemul@openvz.org>
Signed-off-by: Kirill Korotaev <dev@openvz.org>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Zach Brown <zach.brown@oracle.com>
Cc: Davide Libenzi <davidel@xmailserver.org>
Cc: John McCutchan <ttb@tentacle.dhs.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: john stultz <johnstul@us.ibm.com>
Cc: Ram Pai <linuxram@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/fs/dquot.c b/fs/dquot.c
index dcc5a58..3a99584 100644
--- a/fs/dquot.c
+++ b/fs/dquot.c
@@ -474,7 +474,7 @@
 		spin_lock(&dq_list_lock);
 		dirty = &dqopt->info[cnt].dqi_dirty_list;
 		while (!list_empty(dirty)) {
-			dquot = list_entry(dirty->next, struct dquot, dq_dirty);
+			dquot = list_first_entry(dirty, struct dquot, dq_dirty);
 			/* Dirty and inactive can be only bad dquot... */
 			if (!test_bit(DQ_ACTIVE_B, &dquot->dq_flags)) {
 				clear_dquot_dirty(dquot);