loader: ip: Do not call getsecs so much

getsecs is very costly, reuse the values we got before.

Fetching a ~30MB kernel with the tftp command use to take ~26 seconds
and now it's ~18 seconds.

Reviewed by: imp, tsoome
MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D33408

(cherry picked from commit c5f24f5e0db24c8e33e6278a4fbb7e68364ca18d)