make java maps grab locks

The java bpfmap fd cache has some implications:
- this won't protect against two SingleWriterBpfMap instantiations
  for the same map (path) in the same process
- we don't release any locks on BpfMap destruction.

Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: Ibb0b1f57974c307ea5e0fa699ce8d598c59f456e
3 files changed