FREF() earlier in sys_flock()

[Martin Pieuchot]

FREF() is currently needed before any sleep point.  That's because
there's almost no parallelism in the top half of the kernel.  I'd
like to improve that.

The first step is to make sure all operations manipulating a
'struct file *' do so with a properly refcounted element.  So here's
a diff moving FREF() just after fd_getfile() in sys_flock().

Ok?

Index: kern/kern_descrip.c
===================================================================
RCS file: /cvs/src/sys/kern/kern_descrip.c,v
retrieving revision 1.142
diff -u -p -r1.142 kern_descrip.c
--- kern/kern_descrip.c 19 Feb 2018 08:59:52 -0000      1.142
+++ kern/kern_descrip.c 27 Mar 2018 09:12:45 -0000
@@ -1205,9 +1205,11 @@ sys_flock(struct proc *p, void *v, regis
 
        if ((fp = fd_getfile(fdp, fd)) == NULL)
                return (EBADF);
-       if (fp->f_type != DTYPE_VNODE)
-               return (EOPNOTSUPP);
        FREF(fp);
+       if (fp->f_type != DTYPE_VNODE) {
+               error = EOPNOTSUPP;
+               goto out;
+       }
        vp = fp->f_data;
        lf.l_whence = SEEK_SET;
        lf.l_start = 0;