xfs: hoist the node iroot update code out of xfs_btree_kill_iroot

xfs: hoist the node iroot update code out of xfs_btree_kill_iroot

In preparation for allowing records in an inode btree root, hoist the
code that copies keyptrs from an existing node child into the root block
to a separate function.  Remove some unnecessary conditionals and clean
up a few function calls in the new function.  Note that this change
reorders the ->free_block call with respect to the change in bc_nlevels
to make it easier to support inode root leaf blocks in the next patch.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>