nfs: correctly handle mount failures
nfs: correctly handle mount failures

The error reporting and handling in the NFS API is really crappy
anyway, but at least pass the NULL in the right place to prevent a
page fault on mount failures...

mslovy/barrelfish,utsav2601/cmpe295A,utsav2601/cmpe295A,kishoredbn/barrelfish,8l/barrelfish,kishoredbn/barrelfish,BarrelfishOS/barrelfish,BarrelfishOS/barrelfish,8l/barrelfish,kishoredbn/barrelfish,linusyang/barrelfish,linusyang/barrelfish,8l/barrelfish,8l/barrelfish,mslovy/barrelfish,BarrelfishOS/barrelfish,utsav2601/cmpe295A,mslovy/barrelfish,BarrelfishOS/barrelfish,8l/barrelfish,utsav2601/cmpe295A,kishoredbn/barrelfish,BarrelfishOS/barrelfish,linusyang/barrelfish,kishoredbn/barrelfish,kishoredbn/barrelfish,BarrelfishOS/barrelfish,BarrelfishOS/barrelfish,BarrelfishOS/barrelfish,8l/barrelfish,utsav2601/cmpe295A,mslovy/barrelfish,mslovy/barrelfish,utsav2601/cmpe295A,mslovy/barrelfish,linusyang/barrelfish,linusyang/barrelfish,BarrelfishOS/barrelfish