As of commit 9f5c4ef328 ("dounmount(9): temporarily enable recursion
for the covered vnode lock"), the unmount path handles recursion
automatically, so there's no longer a need to handle this case
in unionfs-specific code.

dounmount(): generalize comment on covered vnode lock recursion

The need for recursion applies (in somewhat different form) to both
nullfs and unionfs, and would likely apply to any other hypothetical
stacked filesystem as well.

unionfs_noderem(): fix typo