btrfs: send: do not BUG_ON() on unexpected symlink data extent

There's really no need to BUG_ON() if we find a symlink with an extent
that is not inline or is compressed. We can just make send fail with
an error (-EUCLEAN) and log an informative error message, so just do
that instead of BUG_ON().

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>