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
Sponsored by: The FreeBSD Foundation

(cherry picked from commit b2f9575646f89cdddcad76acae3e9305535506a2)