pflock: add phase-fair reader writer locks

pflock: add phase-fair reader writer locks

This is a new type of reader-writer lock that provides better fairness
guarantees which better suited for typical DPDK applications.
A pflock has two ticket pools, one for readers and one
for writers.

Phase-fair reader writer locks ensure that neither reader nor writer will
be starved.
Neither reader or writer are preferred, they execute in alternating
phases.
All operations of the same type (reader or writer) that acquire the lock
are handled in FIFO order.
Write operations are exclusive, and multiple read operations can be run
together (until a write arrives).

A similar implementation is in Concurrency Kit package in FreeBSD.
For more information see:
   "Reader-Writer Synchronization for Shared-Memory Multiprocessor
    Real-Time Systems",
    http://www.cs.unc.edu/~anderson/papers/ecrts09b.pdf

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>