Consistently protect `llbBridgeForwarder.refs` with mutex
Consistently protect `llbBridgeForwarder.refs` with mutex

Most accesses to this map are protected. Add the missing locks (some of which
are in code paths where we wouldn't normally expect concurrent accesses, but
better to be safe).

Signed-off-by: Ian Campbell <a8db091655a7f47ecb9c39a2d278e9662e440dc0@docker.com>

tonistiigi/buildkit_poc,tonistiigi/buildkit_poc