untangling do_lookup() - isolate !dentry stuff from the rest of it.

Duplicate the revalidation-related parts into if (!dentry) branch.
Next step will be to pull them under i_mutex.

This and the next 8 commits are more or less a splitup of patch
by Miklos; folks, when you are working with something that convoluted,
carve your patches up into easily reviewed steps, especially when
a lot of codepaths involved are rarely hit...
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namei.c

@@ -1194,6 +1194,21 @@ static int do_lookup(struct nameidata *nd, struct qstr *name,
 			status = 1;
 		}
 		mutex_unlock(&dir->i_mutex);
+		if (unlikely(dentry->d_flags & DCACHE_OP_REVALIDATE) && need_reval)
+			status = d_revalidate(dentry, nd);
+		if (unlikely(status <= 0)) {
+			if (status < 0) {
+				dput(dentry);
+				return status;
+			}
+			if (!d_invalidate(dentry)) {
+				dput(dentry);
+				dentry = NULL;
+				need_reval = 1;
+				goto retry;
+			}
+		}
+		goto done;
 	}
 	if (unlikely(dentry->d_flags & DCACHE_OP_REVALIDATE) && need_reval)
 		status = d_revalidate(dentry, nd);
@@ -1209,7 +1224,7 @@ static int do_lookup(struct nameidata *nd, struct qstr *name,
 			goto retry;
 		}
 	}
-
+done:
 	path->mnt = mnt;
 	path->dentry = dentry;
 	err = follow_managed(path, nd->flags);