Prevent race on accessing kmutex_t when the mutex is
embedded in a ref counted structure.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Gvozden Neskovic <neskovic@gmail.com>
Closes zfsonlinux/zfs#6401
Closes#637