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: 9d1ccbe7
Reported-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Jan Kara <jack@suse.cz>