Fix ifdefs for linux

Change-Id: Iaf6fcd0f691e14edc82c9d1286c3d1b8ebeb1237
diff --git a/ext4_utils/make_ext4fs.c b/ext4_utils/make_ext4fs.c
index 266f54e..c96048b 100644
--- a/ext4_utils/make_ext4fs.c
+++ b/ext4_utils/make_ext4fs.c
@@ -31,7 +31,7 @@
 #include <dirent.h>
 #include <libgen.h>
 
-#if defined(linux)
+#if defined(__linux__)
 #include <linux/fs.h>
 #elif defined(__APPLE__) && defined(__MACH__)
 #include <sys/disk.h>
@@ -602,7 +602,7 @@
 	if (fd < 0)
 		return 0;
 
-#if defined(linux)
+#if defined(__linux__)
 	ret = ioctl(fd, BLKGETSIZE64, &size);
 #elif defined(__APPLE__) && defined(__MACH__)
 	ret = ioctl(fd, DKIOCGETBLOCKCOUNT, &size);