make rename() slightly more scalable
make rename() slightly more scalable

if "a" and "b" point to the same inode, rename("a", "b") need not touch
the name for "b".

aclements/sv6,aclements/sv6,bowlofstew/sv6,bowlofstew/sv6,aclements/sv6,aclements/sv6,bowlofstew/sv6,bowlofstew/sv6,aclements/sv6,bowlofstew/sv6