getdirentries: return ENOENT for unlinked but still open directory.

To be more compatible to IEEE Std 1003.1-2008 (“POSIX.1”).

Reviewed by: mjg, Pau Amma (doc)
Differential revision: https://reviews.freebsd.org/D34680
MFC after: 2 weeks

(cherry picked from commit c6487446d7e99537551d2e51a2f6c6569fcb89fc)