powerpc/mm/radix: Fix overuse of small pages in splitting logic

When we have CONFIG_STRICT_KERNEL_RWX enabled, we want to split the
linear mapping at the text/data boundary so we can map the kernel text
read only.

But the current logic uses small pages for the entire text section,
regardless of whether a larger page size would fit. eg. with the
boundary at 16M we could use 2M pages, but instead we use 64K pages up
to the 16M boundary:

  Mapped 0x0000000000000000-0x0000000001000000 with 64.0 KiB pages
  Mapped 0x0000000001000000-0x0000000040000000 with 2.00 MiB pages
  Mapped 0x0000000040000000-0x0000000100000000 with 1.00 GiB pages

This is because the test is checking if addr is < __init_begin
and addr + mapping_size is >= _stext. But that is true for all pages
between _stext and __init_begin.

Instead what we want to check is if we are crossing the text/data
boundary, which is at __init_begin. With that fixed we see:

  Mapped 0x0000000000000000-0x0000000000e00000 with 2.00 MiB pages
  Mapped 0x0000000000e00000-0x0000000001000000 with 64.0 KiB pages
  Mapped 0x0000000001000000-0x0000000040000000 with 2.00 MiB pages
  Mapped 0x0000000040000000-0x0000000100000000 with 1.00 GiB pages

ie. we're correctly using 2MB pages below __init_begin, but we still
drop down to 64K pages unnecessarily at the boundary.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>