btrfs: raid56: no need for irqsafe locking

These days all the operations that take locks in the raid56.c code are
run from user context (mostly workqueues).  Drop all the irqsafe locking
that is not required any more.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>