vfs: flatten vop vectors

This eliminates the following loop from all VOP calls:

while(vop != NULL && \

vop->vop_spare2 == NULL && vop->vop_bypass == NULL)
    vop = vop->vop_default;

Reviewed by: jeff
Tesetd by: pho
Differential Revision: https://reviews.freebsd.org/D22738