btrfs-progs: kerncompat: add local definition for alignment macros

There's still problem left with compilation on musl and kernel < 5.11,
because __ALIGN_KERNEL is not defined anymore:

../bin/ld: kernel-shared/volumes.o: in function `create_chunk':
volumes.c:(.text+0x17f8): undefined reference to `__ALIGN_KERNEL'

Due to the entangled includes and unconditional definition of
__ALIGN_KERNEL, we can't use #ifdef in kerncompat.h to define it
eventually (as kerncompat.h is the first include). Instead add local
definitions of the macros and rename them to avoid name clashes.

Pull-request: #433
Signed-off-by: David Sterba <dsterba@suse.com>

kerncompat.h

@@ -359,7 +359,14 @@ do {					\
 
 /* Alignment check */
 #define IS_ALIGNED(x, a)                (((x) & ((typeof(x))(a) - 1)) == 0)
-#define ALIGN(x, a)		__ALIGN_KERNEL((x), (a))
+
+/*
+ * Alignment, copied and renamed from /usr/include/linux/const.h to work around
+ * issues caused by moving the definition in 5.12
+ */
+#define __ALIGN_KERNEL__(x, a)		__ALIGN_KERNEL_MASK__(x, (typeof(x))(a) - 1)
+#define __ALIGN_KERNEL_MASK__(x, mask)	(((x) + (mask)) & ~(mask))
+#define ALIGN(x, a)		__ALIGN_KERNEL__((x), (a))
 
 static inline int is_power_of_2(unsigned long n)
 {