f2fs: Avoid reading renamed directory if parent does not change

The VFS will not be locking moved directory if its parent does not
change.  Change f2fs rename code to avoid reading renamed directory if
its parent does not change.  Having it uninlined while we are reading
it would cause trouble and we won't be able to rely upon ->i_rwsem
on the directory being renamed in cases that do not alter its parent.

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