Followup to -r344552 in which fsck_ffs checks for a size past the

Followup to -r344552 in which fsck_ffs checks for a size past the
last allocated block of the file and if that is found, shortens the
file to reference the last allocated block thus avoiding having it
reference a hole at its end.

This update corrects an error where fsck_ffs miscalculated the last
logical block of the file when the file contained a large hole.

Reported by:  Jamie Landeg-Jones
Tested by:    Peter Holm
MFC after:    2 weeks
Sponsored by: Netflix