fusefs: set d_off during VOP_READDIR

This allows d_off to be used with lseek to position the file so that
getdirentries(2) will return the next entry. It is not used by
readdir(3).

PR: 253411
Reported by: John Millikin <jmillikin@gmail.com>
Reviewed by: cem
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D28605