ffs: Correct the input size check in sysctl_ffs_fsck()

Make sure we return an error if no input was specified, since
SYSCTL_IN() will report success in that case.

Reported by: KMSAN
Reviewed by: mckusick
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30586