smack: fix logic in smack_inode_init_security function

In principle if this function was called with "value" == NULL and "len"
not NULL it could return different results for the "len" compared to a
case where "name" was not NULL. This is a hypothetical case that does
not exist in the kernel, but it's a logic bug nonetheless.

Signed-off-by: Lukasz Pawelczyk <l.pawelczyk@samsung.com>