fix: add .netns_ok flag + regression test for Linux < 5.14

[acbk2b]

What?

Missing network-namespace support declaration

• .netns_ok = 1 was removed in 219ac06f5542ce25493928522656cf2b3d6b9ae4 and merged into rhel8 via 92e7336822905f182a7d3b7c34cb773fd0752c5a.
• Removing it is correct for Linux >= 5.14, where protocols are assumed to support network namespaces, but older kernels still check it.
• Restore .netns_ok where required for pre-5.14 kernels.
• Upstream reference: Linux 5.14 removal of the netns_ok check.

Fix

• Add .netns_ok = 1 in homa_protocol in homa_plumbing.c
• Add a regression test to catch deletion of the protocol
  • Gated for Linux < 5.14

Why?

.netns_ok is required for Linux < 5.14, which includes RHEL 8. This fix is not needed in main, as Linux >= 5.14 removed .netns_ok entirely

Testing?

Added a regression test which spot checks the inclusion of .netns_ok, and is gated for Linux < 5.14 only. All 800 tests pass