Forward-declare struct mbuf so that this file is less self-insufficient

-- don't depend on garbage in <sys/mount.h>.  mbufs aren't actually
used here either.  They should have been completely removed from filesystem
interfaces when they were removed from the interfaces to convert between
file handles and vnodes.

sys/coda/coda_vfsops.h

@@ -43,6 +43,7 @@ struct cfid {
     ViceFid	cfid_fid;
 };
 
+struct mbuf;
 struct mount;
 
 int coda_vfsopstats_init(void);

sys/fs/coda/coda_vfsops.h

@@ -43,6 +43,7 @@ struct cfid {
     ViceFid	cfid_fid;
 };
 
+struct mbuf;
 struct mount;
 
 int coda_vfsopstats_init(void);