Implement PPC wmb() with sync instead of eieio

Implement PPC wmb() with sync instead of eieio

wmb() for PPC was incorrect defined as an eieio instruction in
libibverbs.  eieio only orders pure I/O memory or a pure system memory
accesses.  In a situation where the device drivers use the d_map
kernel services to share a portion of system memory with an I/O
adapter, we need to use sync() instead. See below link for reference:

http://www.ibm.com/developerworks/eserver/articles/powerpc.html

Signed-off-by: Shirley Ma <xma@us.ibm.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>