buf: Add sysctl flag CTLFLAG_TUN to loader tunable

The sysctl variable 'vfs.unmapped_buf_allowed' 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