ipc/sem.c: replace shared sem_otime with per-semaphore value

ipc/sem.c: replace shared sem_otime with per-semaphore value

sem_otime contains the time of the last semaphore operation that completed
successfully.  Every operation updates this value, thus access from
multiple cpus can cause thrashing.

Therefore the patch replaces the variable with a per-semaphore variable.
The per-array sem_otime is only calculated when required.

No performance improvement on a single-socket i3 - only important
for larger systems.

Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Davidlohr Bueso <davidlohr.bueso@hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>