buf.h: Fix declaration of unmapped_buf

For architectures with a small-data area, the __read_mostly section must
present at the object declaration.

(emaste note: This does not appear to have an affect within FreeBSD, but
may be needed by downstream projects that handle read_mostly /
section(".data.read_mostly") differently.)

Pull Request: https://github.com/freebsd/freebsd-src/pull/608

(cherry picked from commit ede6c6c0434a266a1712ffe807f4e31e229948df)