btrfs: wire up iter_file_splice_write

btrfs implements the iter_write op and thus can use the more efficient
iov_iter based splice implementation.  For now falling back to the less
efficient default is pretty harmless, but I have a pending series that
removes the default, and thus would cause btrfs to not support splice
at all.

Reported-by: Andy Lavr <andy.lavr@gmail.com>
Tested-by: Andy Lavr <andy.lavr@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>