vfs: avoid call to inode_lru_list_del() if possible

inode_lru_list_del() is expensive because of per superblock lru locking,
while some inodes are not in lru list.

Adding a check in iput_final() can speedup pipe/sockets workloads on
SMP.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/inode.c

@@ -1330,6 +1330,7 @@ static void iput_final(struct inode *inode)
 	}
 
 	inode->i_state |= I_FREEING;
+	if (!list_empty(&inode->i_lru))
 		inode_lru_list_del(inode);
 	spin_unlock(&inode->i_lock);