mm/pgtable-generic.c: optimize the VM_BUG_ON condition in pmdp_huge_clear_flush()

The developer will have trouble figuring out why the BUG actually
triggered when there is a complex expression in the VM_BUG_ON.  Because we
can only identify the condition triggered BUG via line number provided by
VM_BUG_ON.  Optimize this by spliting such a complex expression into two
simple conditions.

Link: https://lkml.kernel.org/r/20210203084137.25522-1-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>