Update to v4.17.3 kernel headers.

Test: Builds, boots on a walleye.
Change-Id: I389d8b61ec00ea309e38d1b1a2e0dace48c21edb
diff --git a/libc/kernel/uapi/linux/msdos_fs.h b/libc/kernel/uapi/linux/msdos_fs.h
index 9daf251..60ee22a 100644
--- a/libc/kernel/uapi/linux/msdos_fs.h
+++ b/libc/kernel/uapi/linux/msdos_fs.h
@@ -21,7 +21,9 @@
 #include <linux/types.h>
 #include <linux/magic.h>
 #include <asm/byteorder.h>
+#ifndef SECTOR_SIZE
 #define SECTOR_SIZE 512
+#endif
 #define SECTOR_BITS 9
 #define MSDOS_DPB (MSDOS_DPS)
 #define MSDOS_DPB_BITS 4