Some code (e.g., sbappendaddr)  does

#ifdef MBUFTRACE
                m_claim(m0, sb->sb_mowner);
#endif

but since sys/mbuf.h makes m_claim(m, mowner) a no-op of MBUFTRACE is
not defined, the #ifdef is not necessary. Is there some style or
marker-for-future-work reason to leave the #ifdef/#endif?

Another style nit: when there's an m_foo() mbuf function and an MFOO()
macro, MFOO() is usually an inline version of m_foo().  On those
grounds, how about renaming m_claim() to m_claimm(), as it walks an
entire chain doing MCLAIM() on each mbuf?

A manpage for MBUFTRACE machinery would be very helpful, too, to aid
the rest of us in maintaining mbuf-munging code.