Allow calling bpf_map_update_elem on sockmap and sockhash from a BPF
context. The synchronization required for this is a bit fiddly: we
need to prevent the socket from changing it's state while we add it
to the sockmap, since we rely on getting a callback via
sk_prot->unhash. However, we can't just lock_sock like in
sock_map_sk_acquire because that might sleep. So instead we disable
softirq processing and use bh_lock_sock to prevent further
modification.
Signed-off-by: Lorenz Bauer <lmb@xxxxxxxxxxxxxx>