[XFS] move xfs_bmbt_newroot to common code

xfs_bmbt_newroot is a mostly generic implementation of moving from an
inode root to a real block based root. So move it to xfs_btree.c where it
can use all the nice infrastructure there and make it pointer size
agnostic

The new name for it is xfs_btree_new_iroot, following the old naming but
making it clear we're dealing with the root in inode case here, and to
avoid confusion with xfs_btree_new_root which is used for the not inode
rooted case.

SGI-PV: 985583

SGI-Modid: xfs-linux-melb:xfs-kern:32201a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: Bill O'Donnell <billodo@sgi.com>
Signed-off-by: David Chinner <david@fromorbit.com>