bpf: Retry opening map after initial error
bpf: Retry opening map after initial error

Current code used sync.Once and only attempted opening the map once using
sync.Once. Subsequent tried to open the map would always fail without
ever retrying, i.e. the initial failure always became a permanent failure.

Fixes: #2965

Reported-by: da39a3ee5e6b4b0d3255bfef95601890afd80709@mrdima
Signed-off-by: Thomas Graf <5f50a84c1fa3bcff146405017f36aec1a10a9e38@cilium.io>

cilium/cilium,scanf/cilium,michi-covalent/cilium,tklauser/cilium,scanf/cilium,tgraf/cilium,michi-covalent/cilium,scanf/cilium,tklauser/cilium,michi-covalent/cilium,scanf/cilium,cilium/cilium,tklauser/cilium,scanf/cilium,eloycoto/cilium,scanf/cilium,tgraf/cilium,michi-covalent/cilium,eloycoto/cilium,eloycoto/cilium,eloycoto/cilium,eloycoto/cilium,tgraf/cilium,tklauser/cilium,tgraf/cilium,tgraf/cilium,michi-covalent/cilium,cilium/cilium,tklauser/cilium,cilium/cilium,cilium/cilium,eloycoto/cilium,eloycoto/cilium,scanf/cilium,cilium-team/cilium,cilium-team/cilium,tgraf/cilium