NFS: Fix initialisation of I/O result struct in nfs_pgio_rpcsetup

Initialise the result count to 0 rather than initialising it to the
argument count. The reason is that we want to ensure we record the
I/O stats correctly in the case where an error is returned (for
instance in the layoutstats).

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>