For performance, replace the iint spinlock with rwlock/read_lock.

Eric Paris questioned this change, from spinlocks to rwlocks, saying
"rwlocks have been shown to actually be slower on multi processor
systems in a number of cases due to the cache line bouncing required."

Based on performance measurements compiling the kernel on a cold
boot with multiple jobs with/without this patch, Dmitry Kasatkin
and I found that rwlocks performed better than spinlocks, but very
insignificantly.  For example with total compilation time around 6
minutes, with rwlocks time was 1 - 3 seconds shorter... but always
like that.

Changelog v2:
- new patch taken from the 'allocating iint improvements' patch
Signed-off-by: Dmitry Kasatkin <dmitry.kasatkin@intel.com>
Signed-off-by: Mimi Zohar <zohar@us.ibm.com>