f2fs: remove unmatched zero_user_segment when convert inline dentry

Since the layout of regular dentry block is different from inline dentry
block, zero_user_segment starting from MAX_INLINE_DATA(dir) is not
correct for regular dentry block, besides, bitmap is already copied and
used, so there is no necessary to zero page at all, so just remove the
zero_user_segment is OK.

Signed-off-by: Yunlong Song <yunlong.song@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>