ext4: add missing kfree() on error return path in add_new_gdb()

We added some more error handling in b4097142 "ext4: add error
checking to calls to ext4_handle_dirty_metadata()".  But we need to
call kfree() as well to avoid a memory leak.

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