btrfs: Implement set_extent_bits_nowait

It will be used in a future patch that will require modifying an
extent_io_tree struct under a spinlock.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>