lib/nfs/Connection: fix typo in code comment
lib/nfs/Connection: fix reconnect after mount failure

When mounting had not yet finished, SocketMonitor::IsDefined() was
always false, due to the workaround at the beginning of the function
that calls SocketMonitor::Steal().  This commit drops the IsDefined()
check because it was never necessary and breaks reconnect.
lib/nfs/Connection: add assertion