Fix uninitialized var warning by gcc 4.6

Fix the following warning.

/home/yliu/tux3/user/kernel/dir.c: In function `tux_create_entry':
/home/yliu/tux3/user/kernel/dir.c:46:9: error: `name_len' may be used uninitialized in this function [-Werror=uninitialized]
/home/yliu/tux3/user/kernel/dir.c:130:47: note: `name_len' was declared here

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
[Use uninitialized_var()]
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>