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

[markj]

Summary

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.

Reported by: syzkaller