ARCv2: spinlock: remove the extra smp_mb before lock, after unlock

ARCv2: spinlock: remove the extra smp_mb before lock, after unlock

 - ARCv2 LLSC spinlocks have smp_mb() both before and after the LLSC
   instructions, which is not required per lkmm ACQ/REL semantics.
   smp_mb() is only needed _after_ lock and _before_ unlock.
   So remove the extra barriers.
   The reason they were there was mainly historical. At the time of
   initial SMP Linux bringup on HS38 cores, I was too conservative,
   given the fluidity of both hw and sw. The last attempt to ditch the
   extra barrier showed some hackbench regression which is apparently
   not the case now (atleast for LLSC case, read on...)

 - EX based spinlocks (!CONFIG_ARC_HAS_LLSC) still needs the extra
   smp_mb(), not due to lkmm, but due to some hardware shenanigans.
   W/o that, hackbench triggers RCU stall splat so extra DMB is retained
   !LLSC based systems are not realistic Linux sstem anyways so they can
   afford to be a nit suboptimal ;-)

   | [ARCLinux]# for i in (seq 1 1 5) ; do hackbench; done
   | Running with 10 groups 400 process
   | INFO: task hackbench:158 blocked for more than 10 seconds.
   |       Not tainted 4.20.0-00005-g96b18288a88e-dirty #117
   | "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
   | hackbench       D    0   158    135 0x00000000
   |
   | Stack Trace:
   | watchdog: BUG: soft lockup - CPU#3 stuck for 59s! [hackbench:469]
   | Modules linked in:
   | Path: (null)
   | CPU: 3 PID: 469 Comm: hackbench Not tainted 4.20.0-00005-g96b18288a88e-dirty
   |
   | [ECR   ]: 0x00000000 => Check Programmer's Manual
   | [EFA   ]: 0x00000000
   | [BLINK ]: do_exit+0x4a6/0x7d0
   | [ERET  ]: _raw_write_unlock_irq+0x44/0x5c

 - And while at it, remove the extar smp_mb() from EX based
   arch_read_trylock() since the spin lock there guarantees a full
   barrier anyways

 - For LLSC case, hackbench threads improves with this patch (HAPS @ 50MHz)

   ---- before ----
   |
   | [ARCLinux]# for i in 1 2 3 4 5; do hackbench 10 thread; done
   | Running with 10 groups 400 threads
   | Time: 16.253
   | Time: 16.445
   | Time: 16.590
   | Time: 16.721
   | Time: 16.544

   ---- after ----
   |
   | [ARCLinux]# for i in 1 2 3 4 5; do hackbench 10 thread; done
   | Running with 10 groups 400 threads
   | Time: 15.638
   | Time: 15.730
   | Time: 15.870
   | Time: 15.842
   | Time: 15.729

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>