netns: don't disable BHs when locking "nsid_lock"

When peernet2id() had to lock "nsid_lock" before iterating through the
nsid table, we had to disable BHs, because VXLAN can call peernet2id()
from the xmit path:
  vxlan_xmit() -> vxlan_fdb_miss() -> vxlan_fdb_notify()
    -> __vxlan_fdb_notify() -> vxlan_fdb_info() -> peernet2id().

Now that peernet2id() uses RCU protection, "nsid_lock" isn't used in BH
context anymore. Therefore, we can safely use plain
spin_lock()/spin_unlock() and let BHs run when holding "nsid_lock".

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>