lib/vdso: Add unlikely() hint into vdso_read_begin()

Place the branch with no concurrent write before the contended case.

Performance numbers for Intel(R) Core(TM) i5-6300U CPU @ 2.40GHz
(more clock_gettime() cycles - the better):
| before | after
-----------------------------------
| 150252214 | 153242367
| 150301112 | 153324800
| 150392773 | 153125401
| 150373957 | 153399355
| 150303157 | 153489417
| 150365237 | 153494270
-----------------------------------
avg | 150331408 | 153345935
diff % | 2 | 0
-----------------------------------
stdev % | 0.3 | 0.1

Co-developed-by: Dmitry Safonov <[e-mail]>
Signed-off-by: Andrei Vagin <[e-mail]>
Signed-off-by: Dmitry Safonov <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Tested-by: Vincenzo Frascino <[e-mail]>
Reviewed-by: Vincenzo Frascino <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

include/vdso/helpers.h

···
 {
 	u32 seq;
 
-	while ((seq = READ_ONCE(vd->seq)) & 1)
+	while (unlikely((seq = READ_ONCE(vd->seq)) & 1))
 		cpu_relax();
 
 	smp_rmb();