Corrupted file URI after filefield migration

Because of str_replace() usage in content_migrate_filefield_data_record_alter() all occurrences of file directory path will be removed from the file path and not the only one at the beginning.

For example, when file directory path is just "files" and file path is "files/profiles/my-files/img1.png" the URI becomes "public://pro/my-/img1.png", which is really wrong and we expect "public://profiles/my-files/img1.png".

Patch follows.