xfs: prepare refcount functions to deal with rtrefcountbt

xfs: prepare refcount functions to deal with rtrefcountbt

Prepare the high-level refcount functions to deal with the new realtime
refcountbt and its slightly different conventions.  Provide the ability
to talk to either refcountbt or rtrefcountbt formats from the same high
level code.

Note that we leave the _recover_cow_leftovers functions for a separate
patch so that we can convert it all at once.

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