vfs: allow tail call optimisation in vops in the common case

Most frequently used vops boil down to checking SDT probes, doing the call and
checking again. There is no vop_post/pre in their case but the check after the
call prevents tail call optimisation from taking place. Instead, check once
upfront. Kernels with debug or vops with non-empty vop_post still don't short
circuit.

Reviewed by: kib
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D22739