ext2: Call dquot_writeback_dquots() with s_umount held

ext2_sync_fs() could be called without s_umount semaphore held when
called through ext2_write_super() from __ext2_write_inode(). This
function then calls dquot_writeback_dquots() which relies on s_umount to
be held for protection against other quota operations.

In fact __ext2_write_inode() does not need all the functionality
ext2_write_super() provides. It is enough to just write the superblock.
So use ext2_sync_super() instead.

Fixes: 9d1ccbe70e
Reported-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/ext2/ext2.h

@@ -796,7 +796,8 @@ void ext2_error(struct super_block *, const char *, const char *, ...);
 extern __printf(3, 4)
 void ext2_msg(struct super_block *, const char *, const char *, ...);
 extern void ext2_update_dynamic_rev (struct super_block *sb);
-extern void ext2_write_super (struct super_block *);
+extern void ext2_sync_super(struct super_block *sb, struct ext2_super_block *es,
+			    int wait);
 
 /*
  * Inodes and files operations