unionfs_lock(): eliminate LK_CANRECURSE special-case

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.

Reviewed by: kib, markj
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D58858