btrfs_root_item maintains the ctime for root updates.  This is not part
of vfs_inode.

Since current_time() uses struct inode* as an argument as Linus
suggested, this cannot be used to update root times unless, we modify
the signature to use inode.

Since btrfs uses nanosecond time granularity, it can also use
ktime_get_real_ts directly to obtain timestamp for the root. It is
necessary to use the timespec time api here because the same
btrfs_set_stack_timespec_*() apis are used for vfs inode times as well.
These can be transitioned to using timespec64 when btrfs internally
changes to use timespec64 as well.

Signed-off-by: Deepa Dinamani <deepa.kernel@gmail.com>
Acked-by: David Sterba <dsterba@suse.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David Sterba <dsterba@suse.com>