backport rwsem from Linux 3.11
mainly to add writer lock stealing by Alex Shi and later improvements
by Michael Lespinasse
http://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/log/lib/rwsem.c?h=linux-3.11.y
Optimized ARM RWSEM algorithm
RWSEM implementation for ARM using atomic functions.
Heavily based on arch/sh/include/asm/rwsem.h