https://cgit.openembedded.org/openembedded-core/plain/meta/recipes-devtools/btrfs-tools/btrfs-tools/0001-btrfs-tools-include-linux-const.h-to-fix-build-with-.patch?id=fd8989db25c674ca65389a2d41bf5b447e8794be

From: Bruce Ashfield <bruce.ashfield@gmail.com>
Date: Fri, 18 Jun 2021 11:10:36 -0400
Subject: [PATCH] btrfs-progs: include linux/const.h to fix build with 5.12+
 headers

btrfs-tools compile fails with mips, musl and 5.12+ headers.

The definition of __ALIGN_KERNEL has moved in 5.12+ kernels, so we
add an explicit include of const.h to pickup the macro:

  | make: *** [Makefile:595: mkfs.btrfs] Error 1
  | make: *** Waiting for unfinished jobs....
  | libbtrfs.a(volumes.o): in function `dev_extent_search_start':
  | /usr/src/debug/btrfs-tools/5.12.1-r0/git/kernel-shared/volumes.c:464: undefined reference to `__ALIGN_KERNEL'
  | collect2: error: ld returned 1 exit status

This is safe for older kernel's as well, since the header still
exists, and is valid to include.

Upstream-status: OE Specific [mips64 + musl + libc-headers]

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>
--- a/kerncompat.h
+++ b/kerncompat.h
@@ -30,6 +30,7 @@
 #include <linux/types.h>
 #include <linux/kernel.h>
 #include <stdint.h>
+#include <linux/const.h>
 
 #include <features.h>