netpoll: warn when there are spaces in parameters

v2: update according to Frans' comments.

Currently, if we leave spaces before dst port,
netconsole will silently accept it as 0. Warn about this.

Also, when spaces appear in other places, make them
visible in error messages.

Signed-off-by: WANG Cong <amwang@redhat.com>
Cc: David Miller <davem@davemloft.net>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>