lib/xz: Add fall-through comments to a switch statement

It's good style. I was also told that GCC 7 is more strict and might
give a warning when such comments are missing.

Signed-off-by: Lasse Collin <[e-mail]>
Suggested-by: Andrei Borzenkov <[e-mail]>
Signed-off-by: Jiri Kosina <[e-mail]>

lib/xz/xz_dec_stream.c

···
 			if (ret != XZ_OK)
 				return ret;
 
+		/* Fall through */
+
 		case SEQ_BLOCK_START:
 			/* We need one byte of input to continue. */
 			if (b->in_pos == b->in_size)
···
 			s->temp.pos = 0;
 			s->sequence = SEQ_BLOCK_HEADER;
 
+		/* Fall through */
+
 		case SEQ_BLOCK_HEADER:
 			if (!fill_temp(s, b))
 				return XZ_OK;
···
 
 			s->sequence = SEQ_BLOCK_UNCOMPRESS;
 
+		/* Fall through */
+
 		case SEQ_BLOCK_UNCOMPRESS:
 			ret = dec_block(s, b);
 			if (ret != XZ_STREAM_END)
 				return ret;
 
 			s->sequence = SEQ_BLOCK_PADDING;
+
+		/* Fall through */
 
 		case SEQ_BLOCK_PADDING:
 			/*
···
 			}
 
 			s->sequence = SEQ_BLOCK_CHECK;
+
+		/* Fall through */
 
 		case SEQ_BLOCK_CHECK:
 			if (s->check_type == XZ_CHECK_CRC32) {
···
 
 			s->sequence = SEQ_INDEX_PADDING;
 
+		/* Fall through */
+
 		case SEQ_INDEX_PADDING:
 			while ((s->index.size + (b->in_pos - s->in_start))
 					& 3) {
···
 
 			s->sequence = SEQ_INDEX_CRC32;
 
+		/* Fall through */
+
 		case SEQ_INDEX_CRC32:
 			ret = crc32_validate(s, b);
 			if (ret != XZ_STREAM_END)
···
 
 			s->temp.size = STREAM_HEADER_SIZE;
 			s->sequence = SEQ_STREAM_FOOTER;
+
+		/* Fall through */
 
 		case SEQ_STREAM_FOOTER:
 			if (!fill_temp(s, b))