msdosfs: Fix handling of eofflag in VOP_READDIR

We also need to set it when an end-of-directory marker is reached.

Reported by: vishwin
Reviewed by: kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D51290

(cherry picked from commit 9f6718073382608519912e178e4c313dd61246b3)