mm/huge_memory.c: make get_huge_zero_page() return bool

mm/huge_memory.c: make get_huge_zero_page() return bool

It's guaranteed that huge_zero_page will not be NULL if
huge_zero_refcount is increased successfully.

When READ_ONCE(huge_zero_page) is returned, there must be a
huge_zero_page and it can be replaced with returning
'true' when we do not care about the value of huge_zero_page.

We can thus make it return bool to save READ_ONCE cpu cycles as the
return value is just used to check if huge_zero_page exists.

Link: https://lkml.kernel.org/r/20210318122722.13135-3-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michel Lespinasse <walken@google.com>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: Thomas Hellstrm (Intel) <thomas_os@shipmail.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Wei Yang <richard.weiyang@linux.alibaba.com>
Cc: William Kucharski <william.kucharski@oracle.com>
Cc: Yang Shi <yang.shi@linux.alibaba.com>
Cc: yuleixzhang <yulei.kernel@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>