ipvs: Use min3() in ip_vs_dbg_callid()

There are two motivations for this:

1. It improves readability to my eyes
2. Using nested min() calls results in a shadowed _min1 variable,
   which is a bit untidy. Sparse complained about this.

I have also replaced (size_t)64 with a variable of type size_t and value 64.
This also improves readability to my eyes.

Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>