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
Approved by: re (gjb)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D42113

(cherry picked from commit 31b94065300a76e52c26b8caec7256574d64a777)
(cherry picked from commit 8c4eaac1534e0486bd0fc6816ccb59fc348a5707)