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


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271174 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f