link_path_walk(): be careful when failing with ENOTDIR

In RCU mode we might end up with dentry evicted just we check
that it's a directory.  In such case we should return ECHILD
rather than ENOTDIR, so that pathwalk would be retries in non-RCU
mode.

Breakage had been introduced in commit b18825a7 - prior to that
we were looking at nd->inode, which had been fetched before
verifying that ->d_seq was still valid.  That form of check
would only be satisfied if at some point the pathname prefix
would indeed have resolved to a non-directory.  The fix consists
of checking ->d_seq after we'd run into a non-directory dentry,
and failing with ECHILD in case of mismatch.

Note that all branches since 3.12 have that problem...
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namei.c

@@ -1954,8 +1954,13 @@ static int link_path_walk(const char *name, struct nameidata *nd)
 				continue;
 			}
 		}
-		if (unlikely(!d_can_lookup(nd->path.dentry)))
+		if (unlikely(!d_can_lookup(nd->path.dentry))) {
+			if (nd->flags & LOOKUP_RCU) {
+				if (unlazy_walk(nd, NULL, 0))
+					return -ECHILD;
+			}
 			return -ENOTDIR;
+		}
 	}
 }