Revert "riscv: mm: support Svnapot in huge vmap"

Revert "riscv: mm: support Svnapot in huge vmap"

This reverts commit ce173474cf19fe7fbe8f0fc74e3c81ec9c3d9807.

We cannot correctly deal with NAPOT mappings in vmalloc/vmap because if
some part of a NAPOT mapping is unmapped, the remaining mapping is not
updated accordingly. For example:

ptr = vmalloc_huge(64 * 1024, GFP_KERNEL);
vunmap_range((unsigned long)(ptr + PAGE_SIZE),
	     (unsigned long)(ptr + 64 * 1024));

leads to the following kernel page table dump:

0xffff8f8000ef0000-0xffff8f8000ef1000    0x00000001033c0000         4K PTE N   ..     ..   D A G . . W R V

Meaning the first entry which was not unmapped still has the N bit set,
which, if accessed first and cached in the TLB, could allow access to the
unmapped range.

That's because the logic to break the NAPOT mapping does not exist and
likely won't. Indeed, to break a NAPOT mapping, we first have to clear
the whole mapping, flush the TLB and then set the new mapping ("break-
before-make" equivalent). That works fine in userspace since we can handle
any pagefault occurring on the remaining mapping but we can't handle a kernel
pagefault on such mapping.

So fix this by reverting the commit that introduced the vmap/vmalloc
support.

Fixes: ce173474cf19 ("riscv: mm: support Svnapot in huge vmap")
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20240227205016.121901-2-alexghiti@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>