Correct CTLTYPE of SYSCTL_SBINTIME_MSEC etc

These should be CTLTYPE_S64, not CTLTYPE_INT, since they handle 64-bit
values.

Reviewed by: imp
Fixes: 003ffd57fee1 ("Add sysctl_usec_to_sbintime [...]")
MFC after: 2 weeks
Sponsored by: Amazon
Differential Revision: https://reviews.freebsd.org/D49584