rwsem: Add comments to explain the meaning of the rwsem's count field

It took me quite a while to understand how rwsem's count field
mainifested itself in different scenarios.

Add comments to provide a quick reference to the the rwsem's count
field for each scenario where readers and writers are contending
for the lock.

Hopefully it will be useful for future maintenance of the code and
for people to get up to speed on how the logic in the code works.

Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Davidlohr Bueso <davidlohr@hp.com>
Cc: Alex Shi <alex.shi@linaro.org>
Cc: Michel Lespinasse <walken@google.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: Paul E.McKenney <paulmck@linux.vnet.ibm.com>
Cc: Jason Low <jason.low2@hp.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/1399060437.2970.146.camel@schen9-DESK


Signed-off-by: Ingo Molnar <mingo@kernel.org>