Fix makefs bootstrap after d485c77f203fb0f4cdc08dea5ff81631b51d8809

The makefs msdosfs code includes fs/msdosfs/denode.h which directly uses
struct buf from <sys/buf.h> rather than the makefs struct m_buf.
To work around this problem provide a local denode.h that includes
ffs/buf.h and defines buf as an alias for m_buf.

Reviewed By: kib, emaste
Differential Revision: https://reviews.freebsd.org/D28835