No need to do lock_super() for exclusion in generic_shutdown_super()

We can't run into contention on it.  All other callers of lock_super()
either hold s_umount (and we have it exclusive) or hold an active
reference to superblock in question, which prevents the call of
generic_shutdown_super() while the reference is held.  So we can
replace lock_super(s) with get_fs_excl() in generic_shutdown_super()
(and corresponding change for unlock_super(), of course).

Since ext4 expects s_lock held for its put_super, take lock_super()
into it.  The rest of filesystems do not care at all.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/ext4/super.c

@@ -576,6 +576,7 @@ static void ext4_put_super(struct super_block *sb)
 	struct ext4_super_block *es = sbi->s_es;
 	int i, err;
 
+	lock_super(sb);
 	if (sb->s_dirt)
 		ext4_write_super(sb);
 
@@ -645,7 +646,6 @@ static void ext4_put_super(struct super_block *sb)
 	unlock_super(sb);
 	kobject_put(&sbi->s_kobj);
 	wait_for_completion(&sbi->s_kobj_unregister);
-	lock_super(sb);
 	lock_kernel();
 	kfree(sbi->s_blockgroup_lock);
 	kfree(sbi);