ARM: fix alignment of keystone page table fixup

If init_mm.brk is not section aligned, the LPAE fixup code will miss
updating the final PMD.  Fix this by aligning map_end.

Fixes: a77e0c7b ("ARM: mm: Recreate kernel mappings in early_paging_init()")
Cc: <stable@vger.kernel.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>