devfs: Set eofflag in devfs_readdir()

At least vn_dir_next_dirent() checks that eofflag is set appropriately
when INVARIANTS is enabled, so the omission here could cause an
assertion failure.

Reviewed by: kib
MFC after: 2 weeks
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D50598