Linux 3.16 compat: smp_mb__after_clear_bit()

The smp_mb__{before,after}_clear_bit functions have been renamed
smp_mb__{before,after}_atomic.  Rather than adding a compatibility
function to handle this the code has been updated to use smp_wmb().

This has the advantage of being a stable functionally equivalent
interface.  On many architectures smp_mb__after_clear_bit() expands
to smp_wmb().  Others might be able to do something slightly more
efficient but this will be safe and correct on all of them.

Signed-off-by: Turbo Fredriksson <turbo@bayour.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #386