xfs: hoist the node iroot update code out of xfs_btree_new_iroot

xfs: hoist the node iroot update code out of xfs_btree_new_iroot

In preparation for allowing records in an inode btree root, hoist the
code that copies keyptrs from an existing node root into a child block
to a separate function.  Note that the new function explicitly computes
the keys of the new child block and stores that in the root block; while
the bmap btree could rely on leaving the key alone, realtime rmap needs
to set the new high key.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>