loader: ip: Do not call getsecs so much

[manu]

Summary

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.

MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG

Test Plan

x time-base
+ time-ip-nogetsecs
+-----------------------------------------------------------------------------------------------------------------------------+
| +                                                                                                                           |
| +                                                                                                                           |
| +                                                                                                                           |
| +                                                                                                                           |
|++                                                                                                                           |
|++                                                                                                                           |
|++                                           xxx   x   x x      x                         x  x                              x|
||A                                       |______________M__________A__________________________|                              |
+-----------------------------------------------------------------------------------------------------------------------------+
    N           Min           Max        Median           Avg        Stddev
x  10           104           193           116         128.9     29.856881
+  10            53            54            54          53.7    0.48304589
Difference at 95.0% confidence
	-75.2 +/- 19.8393
	-58.3398% +/- 6.41603%
	(Student's t, pooled s = 21.1148)