Long ago, SPL atomics were implemented as a global spinlock over
conventional operations. In 5e9b5d832b (2009-10) they was converted to
proper atomics, with the spinlock retained as a fallback.
The switch to compile with the fallback was later removed in a91258913f
(2018-05), but the code it enabled wasn't. So lets do that.
Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes#18117