Ensure that fsck(8) / fsck_ffs(8) produces the correct exit code for missing devices.
PR: 262580
Approved by: re (gjb, early MFC)
(cherry picked from commit 2983ec0a87a18943564548c5c00c879c8db83edf)
(cherry picked from commit 934ba51bc929ee23248353a081a4cc4327a4ec54)