pmxcfs: protect CPG operations with mutex

pmxcfs: protect CPG operations with mutex

cpg_mcast_joined (and transitively, cpg_join/leave) are not thread-safe.
pmxcfs triggers such operations via FUSE and CPG dispatch callbacks,
which are running in concurrent threads.

accordingly, we need to protect these operations with a mutex, otherwise
they might return CS_OK without actually doing what they were supposed
to do (which in turn can lead to the dfsm taking a wrong turn and
getting stuck in a supposedly short-lived state, blocking access via
FUSE and getting whole clusters fenced).

huge thanks to Alexandre Derumier for providing the initial bug report
and quite a lot of test runs while debugging this issue.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>