Don't acquire an additional vnode reference to a vnode when it is opened

and then release it when it is closed: we rely on the caller to keep the
vnode around with a valid reference.  This avoids vrele() destroying the
vnode vop_close() is being called from during a call to vop_close(), and
a crash due to lockmgr recursing the vnode lock when a Coda unmount
occurs.

MFC after:	3 days

sys/fs/coda/coda_vnops.c

@@ -221,9 +221,6 @@ coda_open(struct vop_open_args *ap)
 
     CODADEBUG( CODA_OPEN,myprintf(("open: vp %p result %d\n", vp, error));)
 
-    /* Keep a reference until the close comes in. */
-    vref(*vpp);                
-
     /* Save the vnode pointer for the cache file. */
     if (cp->c_ovp == NULL) {
 	cp->c_ovp = vp;
@@ -293,8 +290,6 @@ coda_close(struct vop_close_args *ap)
          error = venus_close(vtomi(vp), &cp->c_fid, flag, cred, td->td_proc);
     else error = ENODEV;
 
-    vrele(vp);
-
     CODADEBUG(CODA_CLOSE, myprintf(("close: result %d\n",error)); )
     return(error);
 }