timer: Fix possible issues with non serialized timer_pending( )

When stress testing ARC Linux from 3.9-rc3, we've hit a serialization
issue when mod_timer() races with itself. This is on a FPGA board and
kernel .config among others has !SMP and !PREEMPT_COUNT.

The issue happens in mod_timer( ) because timer_pending( ) based early
exit check is NOT done inside the timer base spinlock - as a networking
optimization.

The value used in there, timer->entry.next is also used further in call
chain (all inlines though) for actual list manipulation. However if the
register containing this pointer remains live across the spinlock (in a
UP setup with !PREEMPT_COUNT there's nothing forcing gcc to reload) then
a stale value of next pointer causes incorrect list manipulation,
observed with following sequence in our tests.

(0). tv1[x] <----> t1 <---> t2
(1). mod_timer(t1) interrupted after it calls timer_pending()
(2). mod_timer(t2) completes
(3). mod_timer(t1) resumes but messes up the list.
(4). __runt_timers( ) uses bogus timer_list entry / crashes in
     timer->function

The simplest fix is to NOT rely on spinlock based compiler barrier but
add an explicit one in timer_pending()

FWIW, the relevant ARCompact disassembly of mod_timer which clearly
shows the issue due to register reuse is:

mod_timer:
    push_s blink
    mov_s r13,r0	# timer, timer

...
    ###### timer_pending( )
    ld_s r3,[r13]       # <------ <variable>.entry.next LOADED
    brne r3, 0, @.L163

.L163:
....
    ###### spin_lock_irq( )
    lr  r5, [status32]  # flags
    bic r4, r5, 6       # temp, flags,
    and.f 0, r5, 6      # flags,
    flag.nz r4

    ###### detach_if_pending( ) begins

    tst_s r3,r3  <--------------
			# timer_pending( ) checks timer->entry.next
                        # r3 is NOT reloaded by gcc, using stale value
    beq.d @.L169
    mov.eq r0,0

    #  detach_timer( ): __list_del( )

    ld r4,[r13,4]    	# <variable>.entry.prev, D.31439
    st r4,[r3,4]     	# <variable>.prev, D.31439
    st r3,[r4]       	# <variable>.next, D.30246

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Reported-by: Christian Ruppert <christian.ruppert@abilis.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Christian Ruppert <christian.ruppert@abilis.com>
Cc: Pierrick Hascoet <pierrick.hascoet@abilis.com>
Cc: linux-kernel@vger.kernel.org