Fix unmount race condition that can trigger a kernel panic

We should not be using the mount_state field of structure fuse_data without
holding the device lock. Doing so can trigger a kernel panic:

If the FUSE device handle is closed while the corresponding volume is being
unmounted fuse_device_close might release the shared fuse_data structure before
biglock is unlocked in fuse_vfsop_unmount. Then, when attempting to unlock the
already freed biglock in fuse_vfsop_unmount a kernel panic is triggered.

Tracked as issue #77: https://github.com/osxfuse/osxfuse/issues/77
Bump version to 2.5.5