Fix usr.sbin/makefs/makefs_ffs_tests when /etc/fstab does not exist

dumpfs prints a harmless warning message (via ufs_disk_fillout(3) and
getfsfile(3)), when /etc/fstab does not exist. We can ignore it.

PR: 220165
Reported by: gjb
MFC after: 3 weeks
Sponsored by: Spectra Logic Corp