btrfs: uapi: record temporary super flags used by btrfstune

[BUG]
There is a bug report that a canceled checksum conversion (still
experimental feature) results in unexpected super block flags:

csum_type		0 (crc32c)
csum_size		4
csum			0x14973811 [match]
bytenr			65536
flags			0x1000000001
			( WRITTEN |
			  CHANGING_FSID_V2 )
magic			_BHRfS_M [match]

While for a filesystem with ongoing checksum conversion it should have
either CHANGING_DATA_CSUM or CHANGING_META_CSUM.

[CAUSE]
It turns out that, due to btrfs-progs keeps its own extra flags inside
its own ctree.h headers, not the shared uapi headers, we have
conflicting super flags:

kernel-shared/uapi/btrfs_tree.h:#define BTRFS_SUPER_FLAG_METADUMP_V2	(1ULL << 34)
kernel-shared/uapi/btrfs_tree.h:#define BTRFS_SUPER_FLAG_CHANGING_FSID	(1ULL << 35)
kernel-shared/uapi/btrfs_tree.h:#define BTRFS_SUPER_FLAG_CHANGING_FSID_V2 (1ULL << 36)
kernel-shared/ctree.h:#define BTRFS_SUPER_FLAG_CHANGING_DATA_CSUM	(1ULL << 36)
kernel-shared/ctree.h:#define BTRFS_SUPER_FLAG_CHANGING_META_CSUM	(1ULL << 37)

Note that CHANGING_FSID_V2 is conflicting with CHANGING_DATA_CSUM.

[FIX]
The proper fix would be done inside btrfs-progs, but to keep everything
properly recorded, we should have everything inside the same uapi
header.

Copy all the new flags into uapi header, and change the value for
CHANGING_DATA_CSUM and CHANGING_META_CSUM, while keep the value of
CHANGING_BG_TREE untouched.

Thankfully checksum change is still only experimental and all those
CHANGING_* flags are transient (only for btrfs-progs to resume the
conversion, and kernel will reject them all), the damage is still minor.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

include/uapi/linux/btrfs_tree.h

@@ -777,6 +777,14 @@ struct btrfs_stripe_extent {
 #define BTRFS_SUPER_FLAG_CHANGING_FSID	(1ULL << 35)
 #define BTRFS_SUPER_FLAG_CHANGING_FSID_V2 (1ULL << 36)
 
+/*
+ * Those are temporaray flags utilized by btrfs-progs to do offline conversion.
+ * They are rejected by kernel.
+ * But still keep them all here to avoid conflicts.
+ */
+#define BTRFS_SUPER_FLAG_CHANGING_BG_TREE	(1ULL << 38)
+#define BTRFS_SUPER_FLAG_CHANGING_DATA_CSUM	(1ULL << 39)
+#define BTRFS_SUPER_FLAG_CHANGING_META_CSUM	(1ULL << 40)
 
 /*
  * items in the extent btree are used to record the objectid of the