Merge tag 'nfs-for-3.19-1' into nfsd for-3.19 branch

Mainly what I need is 860a0d9e "sunrpc: add some tracepoints in
svc_rqst handling functions", which subsequent server rpc patches from
jlayton depend on.  I'm merging this later tag on the assumption that's
more likely to be a tested and stable point.