btrfs: remove redundant memory barrier from extent_io_tree_release()

The memory barrier at extent_io_tree_release() is redundant. Holding
spin_lock here is not enough to drop the barrier completely.  We only
change the waitqueue of an extent state record while holding the tree
lock - see wait_on_state().

The update to waitqueue state will not become stale because there will
be an spin_unlock/spin_lock sequence between the change and waiting,
this implies a full memory barrier.

So remove the explicit smp_mb() barrier.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ reword reasoning ]
Signed-off-by: David Sterba <dsterba@suse.com>