x86/mm: Remove pointless checks in vmalloc_fault

vmalloc_fault() sets user's pgd or p4d from the kernel page table.  Once
it's set, all tables underneath are identical. There is no point of
following the same page table with two separate pointers and make sure they
see the same with BUG().

Remove the pointless checks in vmalloc_fault(). Also rename the kernel
pgd/p4d pointers to pgd_k/p4d_k so that their names are consistent in the
file.

Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Toshi Kani <toshi.kani@hpe.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mm@kvack.org
Cc: Borislav Petkov <bp@alien8.de>
Cc: Gratian Crisan <gratian.crisan@ni.com>
Link: https://lkml.kernel.org/r/20180314205932.7193-1-toshi.kani@hpe.com