rwsem-spinlock: let rwsem write lock stealable

We(Linux Kernel Performance project) found a regression introduced by
commit 5a50508, which just convert all mutex lock to rwsem write lock.
The semantics is same, but the results is quite huge in some cases.
After investigation, we found the root cause: mutex support lock
stealing. Here is the link for the detailed regression report:
    https://lkml.org/lkml/2013/1/29/84

Ingo suggests to add write lock stealing to rwsem as well:
    "I think we should allow lock-steal between rwsem writers - that
     will not hurt fairness as most rwsem fairness concerns relate to
     reader vs. writer fairness"

And here is the rwsem-spinlock version.

With this patch, we got a double performance increase in one test box
with following aim7 workfile:
    FILESIZE: 1M
    POOLSIZE: 10M
    10 fork_test

some /usr/bin/time output w/o patch      some /usr/bin/time_output with patch
----------------------------------------------------------------------------
Percent of CPU this job got: 369%        Percent of CPU this job got: 537%
Voluntary context switches: 640595016    Voluntary context switches: 157915561
----------------------------------------------------------------------------
You will see we got a 45% increase of CPU usage and saves about 3/4
voluntary context switches.
Here is the .nr_running filed for all CPUs from /proc/sched_debug.

output w/o this patch:
----------------------
cpu 00:   0   0   ...   0   0   0   0   0   0   0   1   0   1 .... 0   0
cpu 01:   0   0   ...   1   0   0   0   0   0   1   1   0   1 .... 0   0
cpu 02:   0   0   ...   1   1   0   0   0   1   0   0   1   0 .... 1   1
cpu 03:   0   0   ...   0   1   0   0   0   1   1   0   1   1 .... 0   0
cpu 04:   0   1   ...   0   0   2   1   1   2   1   0   1   0 .... 1   0
cpu 05:   0   1   ...   0   0   2   1   1   2   1   1   1   1 .... 0   0
cpu 06:   0   0   ...   2   0   0   1   0   0   1   0   0   0 .... 0   0
cpu 07:   0   0   ...   2   0   0   0   1   0   1   1   0   0 .... 1   0
cpu 08:   0   0   ...   1   0   0   0   1   0   0   1   0   0 .... 0   1
cpu 09:   0   0   ...   1   0   0   0   1   0   0   1   0   0 .... 0   1
cpu 10:   0   0   ...   0   0   0   2   0   0   1   0   1   1 .... 1   2
cpu 11:   0   0   ...   0   0   0   2   2   0   1   0   1   0 .... 1   2
cpu 12:   0   0   ...   2   0   0   0   1   1   3   1   1   1 .... 1   0
cpu 13:   0   0   ...   2   0   0   0   1   1   3   1   1   0 .... 1   1
cpu 14:   0   0   ...   0   0   0   2   0   0   1   1   0   0 .... 1   0
cpu 15:   0   0   ...   1   0   0   2   0   0   1   1   0   0 .... 0   0
output with this patch:
-----------------------
cpu 00:   0   0   ...   1   1   2   1   1   1   2   1   1   1 .... 1   3
cpu 01:   0   0   ...   1   1   1   1   1   1   2   1   1   1 .... 1   3
cpu 02:   0   0   ...   2   2   3   2   0   2   1   2   1   1 .... 1   1
cpu 03:   0   0   ...   2   2   3   2   1   2   1   2   1   1 .... 1   1
cpu 04:   0   1   ...   2   0   0   1   0   1   3   1   1   1 .... 1   1
cpu 05:   0   1   ...   2   0   1   1   0   1   2   1   1   1 .... 1   1
cpu 06:   0   0   ...   2   1   1   2   0   1   2   1   1   1 .... 2   1
cpu 07:   0   0   ...   2   1   1   2   0   1   2   1   1   1 .... 2   1
cpu 08:   0   0   ...   1   1   1   1   1   1   1   1   1   1 .... 0   0
cpu 09:   0   0   ...   1   1   1   1   1   1   1   1   1   1 .... 0   0
cpu 10:   0   0   ...   1   1   1   0   0   1   1   1   1   1 .... 0   0
cpu 11:   0   0   ...   1   1   1   0   0   1   1   1   1   2 .... 1   0
cpu 12:   0   0   ...   1   1   1   0   1   1   0   0   0   1 .... 2   1
cpu 13:   0   0   ...   1   1   1   0   1   1   1   0   1   2 .... 2   0
cpu 14:   0   0   ...   2   0   0   0   0   1   1   1   1   1 .... 2   2
cpu 15:   0   0   ...   2   0   0   1   0   1   1   1   1   1 .... 2   2
------------------------------------------------------------------------
Where you can see that CPU is much busier with this patch.
v2: make it stealable at __down_write_trylock as well, pointed by Michel

Reported-by: LKP project <lkp@linux.intel.com>
Suggested-by: Ingo Molnar <mingo@kernel.org>
Cc: David Howells <dhowells@redhat.com>
Cc: Michel Lespinasse <walken@google.com>
Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
backported for Grouper Kernel from LKML reference

Signed-off-by: faux123 <reioux@gmail.com>