Use refcount_dec_and_test() on 4.16+ kernels, atomic_dec_and_test()
on older kernels. https://lwn.net/Articles/714974/
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes: #7179Closes: #7211
