udf: avoid unneeded up_write when fail to add entry in ->symlink

We have released the ->i_data_sem before invoking udf_add_entry(),
so in following error path, we should not release this lock again.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jan Kara <jack@suse.cz>