vfs: Make __fsync_super() a static function (version 4)

__fsync_super() does the same thing as fsync_super(). So change the only
caller to use fsync_super() and make __fsync_super() static. This removes
unnecessarily duplicated call to sync_blockdev() and prepares ground
for the changes to __fsync_super() in the following patches.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>