Fix layer store Get locking

Get was calling getReference without layerL held. This meant writes to
the references map could race. Such races are dangerous because they can
corrupt the map and crash the process.

Fixes #21616
Fixes #21674

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>
(cherry picked from commit 0538981c31a714a4183e846a4b512deb7879cc29)