[PATCH] fix fs->lock deadlock with emulated name lookup

The patch below is needed to avoid a deadlock on fs->lock.  Without
the patch, if __emul_lookup_dentry() returns 0, we fail to reacquire
current->fs->lock and then go ahead to read_unlock() it anyhow.  Bad
for your health.

I believe the bug was introduced when the fast pathwalk was reverted
in order to introduce the RCU lockless path walking.

fs/namei.c

@@ -847,6 +847,7 @@ int path_lookup(const char *name, unsigned int flags, struct nameidata *nd)
 			read_unlock(&current->fs->lock);
 			if (__emul_lookup_dentry(name,nd))
 				return 0;
+			read_lock(&current->fs->lock);
 		}
 		nd->mnt = mntget(current->fs->rootmnt);
 		nd->dentry = dget(current->fs->root);