FREF() for namei()

[Martin Pieuchot]

namei() currently uses fd_getfile() without calling FREF()/FRELE()
because it doesn't sleep before grabbing a reference on the vnode.

This won't be enough as soon as we start unlocking some syscalls,
so add a FREF()/FRELE() dance.

Ok?

Index: kern/vfs_lookup.c
===================================================================
RCS file: /cvs/src/sys/kern/vfs_lookup.c,v
retrieving revision 1.65
diff -u -p -r1.65 vfs_lookup.c
--- kern/vfs_lookup.c   29 Aug 2017 02:51:27 -0000      1.65
+++ kern/vfs_lookup.c   3 Apr 2018 13:33:35 -0000
@@ -190,12 +190,15 @@ fail:
                        pool_put(&namei_pool, cnp->cn_pnbuf);
                        return (EBADF);
                }
+               FREF(fp);
                dp = (struct vnode *)fp->f_data;
                if (fp->f_type != DTYPE_VNODE || dp->v_type != VDIR) {
+                       FRELE(fp, p);
                        pool_put(&namei_pool, cnp->cn_pnbuf);
                        return (ENOTDIR);
                }
                vref(dp);
+               FRELE(fp, p);
        }
        for (;;) {
                if (!dp->v_mount) {