bzip2/lzma: comprehensible error messages for missing decompressor

Instead of failing to identify a compressed image with a decompressor
that we don't have compiled in, identify it and fail with a
comprehensible panic message.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

init/do_mounts_rd.c

@@ -79,9 +79,12 @@ identify_ramdisk_image(int fd, int start_block, decompress_fn *decompressor)
 	sys_read(fd, buf, size);
 
 	*decompressor = decompress_method(buf, size, &compress_name);
-	if (*decompressor) {
+	if (compress_name) {
 		printk(KERN_NOTICE "RAMDISK: %s image found at block %d\n",
 		       compress_name, start_block);
+		if (!*decompressor)
+			printk(KERN_CRIT "RAMDISK: %s decompressor not configured!\n",
+			       compress_name);
 		nblocks = 0;
 		goto done;
 	}