rcu: Replace list_first_entry_rcu() with list_first_or_null_rcu()

The list_first_entry_rcu() macro is inherently unsafe because it cannot
be applied to an empty list.  But because RCU readers do not exclude
updaters, a list might become empty between the time that list_empty()
claimed it was non-empty and the time that list_first_entry_rcu() is
invoked.  Therefore, the list_empty() test cannot be separated from the
list_first_entry_rcu() call.  This commit therefore combines these to
macros to create a new list_first_or_null_rcu() macro that replaces
the old (and unsafe) list_first_entry_rcu() macro.

This patch incorporates Paul's review comments on the previous version of
this patch available here:

https://lkml.org/lkml/2012/4/2/536



This patch cannot break any upstream code because list_first_entry_rcu()
is not being used anywhere in the kernel (tested with grep(1)), and any
external code using it is probably broken as a result of using it.

Signed-off-by: Michel Machado <michel@digirati.com.br>
CC: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
CC: Dipankar Sarma <dipankar@in.ibm.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>