ext4: Fix NULL dereference in ext4_ext_migrate()'s error handling

This was found through a code checker (http://repo.or.cz/w/smatch.git/

). 
It looks like you might be able to trigger the error by trying to migrate 
a readonly file system.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>