When the last reference on the vnode' vm object is dropped, read the
vp->v_vflag without taking vnode lock and without bypass. We do know
that vp is the lowest level in the stack, since the pointer is
obtained from the object' handle. Stale VV_TEXT flag read can only
happen if parallel execve() is performed and not yet activated the
image, since process takes reference for text mapping. In this case,
the execve() code manages the VV_TEXT flag on its own already.

It was observed that otherwise read-only sendfile(2) requires
exclusive vnode lock and contending on it on some loads for VV_TEXT
handling.

Reported by: glebius, scottl
Tested by: glebius, pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week