(and remove the old straight from fd constructor)
Note that BpfMapRO is not yet truly compile time read-only,
since it still has methods that can modify things (although those
modifications will of course fail due to the map fd being r/o).
Test: build, atest with followup fixes
Bug: 129773125
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I70b511e3f0a62cd4d9d5a923e658b1086337ec4c