nfscl: Add sysctl flag CTLFLAG_TUN to loader tunable

The sysctl variable 'vfs.nfs.iodmin' is actually a loader tunable. Add
sysctl flag CTLFLAG_TUN to it so that sysctl -T will report it
correctly.

No functional change intended.

Reviewed by: kib, imp
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D42113