vfs cache: Simplify cache_enter_time() a bit

The condition flag == NFC_ISDOTDOT && vp != NULL && vp->v_type != VDIR
is never true at this point in the function. This is asserted slightly
earlier. So, remove some dead code and simplify control flow.

N.B. we set v_cache_dd for all vnode types, not just VDIR. This seems
to be intentional, see commit ce575cd0e2f9069. For regular files it
appears to effectively represent the most recently entered cache entry
for the vnode.

No functional change intended.

Reviewed by: olce, kib
MFC after: 2 weeks
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D50107

(cherry picked from commit 01435e28211220f985c66569f60939e440f0887f)