[IPVS]: Make sure ip_vs_ftp ports are valid: module_param_array approach

I'm not entirely sure what happens in the case of a valid port,
at best it'll be silently ignored. This patch ensures that
the port values are unsigned short values, and thus always valid.

This is a second take at fixing this problem, it is simpler
and arguably more correct than the previous approach
that was committed as 3f5af5b353ca36aca4f8a46e3da2172f669dbbbc.
Prior to this patch a patch that reverses
3f5af5b353ca36aca4f8a46e3da2172f669dbbbc was sent.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>