tarfs: Inherit mnt_iosize_max from the lower vnode

There is no obvious reason to use a value smaller than that.

Reviewed by: des, kib
MFC after: 1 week
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D44627

(cherry picked from commit 91eca18551554b7aca80fcfd3c648f524b321252)