tomoyo: Fix null pointer check

Since tomoyo_memory_ok() will check for null pointer returned by
kzalloc() in tomoyo_assign_profile(), tomoyo_assign_namespace(),
tomoyo_get_name() and tomoyo_commit_ok(), then emit OOM warnings
if needed. And this is the expected behavior as informed by
Tetsuo Handa.

Let's add __GFP_NOWARN to kzalloc() in those related functions.
Besides, to achieve this goal, remove the null check for entry
right after kzalloc() in tomoyo_assign_namespace().

Reported-by: Hulk Robot <hulkci@huawei.com>
Suggested-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>