xfs: add a inode btree blocks counts to the AGI header

Years ago, Christoph diagnosed a problem where freeing an inode on a
totally full filesystem could fail due to finobt expansion not being
able to allocate enough blocks.  He solved the problem by using the
per-AG block reservation system to ensure that there are always enough
blocks for finobt expansion, but that came at the cost of having to walk
the entire finobt at mount time.  This new feature solves that
performance regression by adding inode btree block counts to the AGI
header.