Fix kernel panic when invoking fuse_purge_np

HNodeLookupRealQuickIfExists tries to unlock biglock even though it has not
been locked.

Stack trace of the panic:

    fuse_purge_np
    ...
    fuse_device_ioctl
    HNodeLookupRealQuickIfExists
    fusefs_recursive_lock_unlock

Reported by megrimes@socket.net