Correct IE Recovery Time Stamp format

[louisroyer]

• Bug report: https://forum.free5gc.org/t/upf-recovery-time-stamp-value-bug-report/1545

TS 29.244 defines IE Recovery Time Stamp as follow:

The Recovery Time Stamp IE is coded as shown in Figure 8.2.65-1. It indicates the UTC time when the PFCP entity
started. Octets 5 to 8 are encoded in the same format as the first four octets of the 64-bit timestamp format as defined in
clause 6 of IETF RFC 5905.

The current implementation was using Unix time (starting at 1970-01-01) instead of RFC5905 time (starting at 1900-01-01).

The fix consists to add the well known (stated in the RFC) difference in seconds between 1970 and 1900 to the recoveryTime.

[free5gc-org]

This pull request has been mentioned on free5GC. There might be relevant details there:

http://forum.free5gc.org/t/upf-recovery-time-stamp-value-bug-report/1545/2