Clarify a diagnostic printf() in the mbuf code: M_EXT doesn't necessarily
imply a cluster is attached; it could also refer to some other sort of
external storage (e.g., an sf_buf).

MFC after:	3 days
Sponsored by:	EMC / Isilon Storage Division
Garbage collect NFSMINOFF() from the NFS stack; this unused macro replicates
mbuf-initialisation logic that is best left to centralised mbuf utility
code rather than scattered around the kernel.

MFC after:	3 days
Sponsored by:	EMC / Isilon Storage Division