btrfs: add support for SUPER_FLAG_CHANGING_FSID

The UUID change by btrfstune sets SUPER_FLAG_CHANGING_FSID and resets it
only when changing fsid is complete. Its not a good idea to mount the
device anything in between, reading metadata blocks would fail with UUID
mismatch.

This patch doesn't add SUPER_FLAG_CHANGING_FSID into
BTRFS_SUPER_FLAG_SUPP list, so mount will fail (along with the fix in
the next patch) when SUPER_FLAG_CHANGING_FSID is set.
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ update changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>

include/uapi/linux/btrfs_tree.h

@@ -457,6 +457,7 @@ struct btrfs_free_space_header {
 #define BTRFS_SUPER_FLAG_SEEDING	(1ULL << 32)
 #define BTRFS_SUPER_FLAG_METADUMP	(1ULL << 33)
 #define BTRFS_SUPER_FLAG_METADUMP_V2	(1ULL << 34)
+#define BTRFS_SUPER_FLAG_CHANGING_FSID	(1ULL << 35)
 
 
 /*