Ocfs2/refcounttree: Publicize couple of funcs from refcounttree.c

The original goal of commonizing these funcs is to benefit defraging/extent_moving
codes in the future,  based on the fact that reflink and defragmentation having
the same Copy-On-Wrtie mechanism.

Signed-off-by: Tristan Ye <tristan.ye@oracle.com>