pfctl: improve error reporting for routehost

If an invalid (i.e. overly long) interface name is specified error out
immediately, rather than in expand_rule() so we point at the incorrect
line.

PR: 260958
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D34008

(cherry picked from commit e68de6694381748b7578703b22580c0f17780b32)