ufs: Update *eofflag upon a read of an unlinked directory

If the directory is unlinked, no further entries will be returned, but
we return no error. At least one caller (vn_dir_next_dirent()) asserts
that a VOP_READDIR call which returns no error and no entries will set
*eofflag != 0, so the current behaviour of UFS can trigger an assertion
failure.

Simply set *eofflag in this scenario.

Reviewed by: olce, kib
Reported by: syzkaller
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D43089

(cherry picked from commit 3ff574c5e1d1d5d07763a14f22d6f9d7291550c6)