btrfs-progs: set generation_v2 any time we write a new root

btrfs-progs: set generation_v2 any time we write a new root

With this integration branch commit in place:

2bd1169 btrfs-progs: root_item generation_v2 is out of sync after btrfsck

I started seeing generation mismatch messages from the kernel
at mount time, after a fresh mkfs(!):

  btrfs: mismatching generation and generation_v2 found in root item...

This is because the code which emits the warning does not do so if
there is a mismatch but generation_v2 is 0; the above commit began
setting generation_v2 to something non-zero, so the warning was emitted.

The reason there is a mismatch at all is because mkfs.btrfs calls
create_data_reloc_tree(), which copies a root, and then calls
btrfs_set_root_generation(), bumping the original copied generation.
But nothing updated generation_v2 to match on the way to disk.

Fix this by updating generation_v2 in btrfs_insert_root(),
as is done in the kernel.

This is safe because it's a new root created by userspace, so
the btrfs_root_item is guaranteed to be big enough to contain
generation_v2.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>