VFS: add VOP_GETLOWVNODE()

It is similar to VOP_GETWRITEMOUNT(), and for given vnode vp should
return the lower vnode which would actually handle write to vp.
Flags allow to specify FREAD or FWRITE for benefit of possible unionfs
implementation.

Reviewed by: markj, Olivier Certner <olce.freebsd@certner.fr>
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D42603