vfs: compute_lk_cnflags(): Remove unused argument 'cnflags'; Rename

Argument unused since commit 93a0ba8f4990785f.

Rename it to enforce_lkflags(), which seems to more aptly describe what it does.

[mjg: massaged the commit message a little]
Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D40848