xfs: encode the btree geometry flags in the btree ops structure
Certain btree flags never change for the life of a btree cursor because
they describe the geometry of the btree itself. Encode these in the
btree ops structure and reduce the amount of code required in each btree
type's init_cursor functions. This also frees up most of the bits in
bc_flags.
A previous version of this patch also converted the open-coded flags
logic to helpers. This was removed due to the pending refactoring (that
follows this patch) to eliminate most of the state flags.