arm64: Reserve an extra page for early kernel mapping

In the Linux kernel, the following vulnerability has been resolved:

arm64: Reserve an extra page for early kernel mapping

The final part of [data, end) segment may overflow into the next page of
init_pg_end[1] which is the gap page before early_init_stack[2]:

[1]
crash_arm64_v9.0.1> vtop ffffffed00601000
VIRTUAL PHYSICAL
ffffffed00601000 83401000

PAGE DIRECTORY: ffffffecffd62000
PGD: ffffffecffd62da0 => 10000000833fb003
PMD: ffffff80033fb018 => 10000000833fe003
PTE: ffffff80033fe008 => 68000083401f03
PAGE: 83401000

PTE PHYSICAL FLAGS
68000083401f03 83401000 (VALID|SHARED|AF|NG|PXN|UXN)

PAGE PHYSICAL MAPPING INDEX CNT FLAGS
fffffffec00d0040 83401000 0 0 1 4000 reserved

[2]
ffffffed002c8000 (r) __pi__data
ffffffed0054e000 (d) __pi___bss_start
ffffffed005f5000 (b) __pi_init_pg_dir
ffffffed005fe000 (b) __pi_init_pg_end
ffffffed005ff000 (B) early_init_stack
ffffffed00608000 (b) __pi__end

For 4K pages, the early kernel mapping may use 2MB block entries but the
kernel segments are only 64KB aligned. Segment boundaries that fall
within a 2MB block therefore require a PTE table so that different
attributes can be applied on either side of the boundary.

KERNEL_SEGMENT_COUNT still correctly counts the five permanent kernel
VMAs registered by declare_kernel_vmas(). However, since commit
5973a62efa34 ("arm64: map [_text, _stext) virtual address range
non-executable+read-only"), the early mapper also maps [_text, _stext)
separately from [_stext, _etext). This adds one more early-only split
and can require one more page-table page than the existing
EARLY_SEGMENT_EXTRA_PAGES allowance reserves.

Increase the 4K-page early mapping allowance by one page to cover that
additional split.

[catalin.marinas@arm.com: rewrote part of the commit log]
[catalin.marinas@arm.com: expanded the code comment]