zfs: fall back if block_cloning feature is disabled

If block_cloning is disabled, or other errors from zfs_clone_range()
return an EXDEV we should fall back to vn_generic_copy_file_range().

This fixes issues when copying files on the same dataset with
block_cloning disabled.

Upstreamed as pull request to OpenZFS.

Reviewed by: Mateusz Guzik <mjguzik@gmail.com>
OpenZFS pull request: 14713