o arp: fix seq_file handling bug

When midnigth commander viewer is invoked it first opens the file, read
4 bytes (probably looking for a magic number), reading only 4 bytes makes
state->is_pneigh not to be set neither the lock is taken, because only
the header is being produced (v = (void *)1), so when arp_seq_stop is 
called the lock is dropped without having being taken: b00m

Thanks to Serge Kuznetsov for reporting this to me. Other seq_file
code may have this problem, but by using mc viewer in all of /proc/net
I haven't been able to reproduce this problem with any other file.

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