btrfs: set generation before calling btrfs_clean_tree_block in btrfs_init_new_buffer

In the Linux kernel, the following vulnerability has been resolved:

btrfs: set generation before calling btrfs_clean_tree_block in btrfs_init_new_buffer

syzbot is reporting uninit-value in btrfs_clean_tree_block() [1], for

commit bc877d285ca3dba2 ("btrfs: Deduplicate extent_buffer init code")

missed that btrfs_set_header_generation() in btrfs_init_new_buffer() must

not be moved to after clean_tree_block() because clean_tree_block() is

calling btrfs_header_generation() since commit 55c69072d6bd5be1 ("Btrfs:

Fix extent_buffer usage when nodesize != leafsize").

Since memzero_extent_buffer() will reset "struct btrfs_header" part, we

can't move btrfs_set_header_generation() to before memzero_extent_buffer().

Just re-add btrfs_set_header_generation() before btrfs_clean_tree_block().