Add compiler barriers around modifications of the robust mutex list for...

Add compiler barriers around modifications of the robust mutex list for pthread_mutex_trylock. [BZ #24180]

While debugging a kernel warning, Thomas Gleixner, Sebastian Sewior and
Heiko Carstens found a bug in pthread_mutex_trylock due to misordered
instructions:
140:   a5 1b 00 01             oill    %r1,1
144:   e5 48 a0 f0 00 00       mvghi   240(%r10),0   <--- THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL);
14a:   e3 10 a0 e0 00 24       stg     %r1,224(%r10) <--- last THREAD_SETMEM of ENQUEUE_MUTEX_PI

vs (with compiler barriers):
140:   a5 1b 00 01             oill    %r1,1
144:   e3 10 a0 e0 00 24       stg     %r1,224(%r10)
14a:   e5 48 a0 f0 00 00       mvghi   240(%r10),0

Please have a look at the discussion:
"Re: WARN_ON_ONCE(!new_owner) within wake_futex_pi() triggerede"
(https://lore.kernel.org/lkml/20190202112006.GB3381@osiris/)

This patch is introducing the same compiler barriers and comments
for pthread_mutex_trylock as introduced for pthread_mutex_lock and
pthread_mutex_timedlock by commit 8f9450a0
"Add compiler barriers around modifications of the robust mutex list."

ChangeLog:

	[BZ #24180]
	* nptl/pthread_mutex_trylock.c (__pthread_mutex_trylock):
	Add compiler barriers and comments.

(cherry picked from commit 823624bd)