pnfs/blocklayout: serialize GETDEVICEINFO calls

The rpc_pipefs code isn't thread safe, leading to occasional use after
frees when running xfstests generic/241 (dbench).

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: http://lkml.kernel.org/r/1411740170-18611-2-git-send-email-hch@lst.de


Cc: stable@vger.kernel.org # 3.17.x
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>