grep: Suppress EISDIR when reading.

MFC after: 1 week
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
Reviewed by: kevans
Approved by: re (delphij)

(cherry picked from commit caf5283fc92edb83173752b1dafbc6d586e78815)
(cherry picked from commit 24b741db5fc907d35e51c97f8ef8ad595474e6b9)