Fix a bug in fsck_ffs(8) triggered by corrupted filesystems.

Check for valid file size before processing journal entries for it.
Done by extracting the file size check from pass1.c into chkfilesize()
then using it in the journal code in suj.c

Reported-by: Robert Morris
PR: 271378
MFC-after: 1 week
Sponsored-by: The FreeBSD Foundation