VFS: Add zfs_inode_update() helper

VFS: Add zfs_inode_update() helper

For the moment we have left ZFS unchanged and it updates many values
as part of the znode.  However, some of these values should be set
in the inode.  For the moment this is handled by adding a function
called zfs_inode_update() which updates the inode based on the znode.

This is considered a workaround until we can systematically go
through the ZFS code and have it directly update the inode.  At
which point zfs_update_inode() can be dropped entirely.  Keeping
two copies of the same data isn't only inefficient it's a breeding
ground for bugs.