xfs: refactor btrees to support records in inode root

This series prepares the btree code to support realtime reverse mapping
btrees by refactoring xfs_ifork_realloc to be fed a per-btree ops
structure so that it can handle multiple types of inode-rooted btrees.
It moves on to refactoring the btree code to use the new realloc
routines and to support storing btree rcords in the inode root, because
the current bmbt code does not support this.