gencgc: Fix potential out-of-bounds access in page_ends_contiguous_block_p().

gencgc: Fix potential out-of-bounds access in page_ends_contiguous_block_p().

  * If we're testing to see if the LAST page in dynamic space is
the end of a contiguous block, and it is a full page (bytes_used
is GENCGC_CARD_BYTES), we turn around and start investigating the
next page table entry... but there isn't one, it's beyond the end
of the allocation.

  * Fix, by bounds-testing the page index against the index of the
high-water mark for dynamic space.  This is guaranteed to be no
more than the total maximum for the page table, and is slightly
more micro-efficient than using the actual maximum, as any page
after the high-water mark will be page_free_p().