x86/mm: Expand static page table for fixmap space

We met a kernel panic when enabling earlycon, which is due to the fixmap
address of earlycon is not statically setup.

Currently the static fixmap setup in head_64.S only covers 2M virtual
address space, while it actually could be in 4M space with different
kernel configurations, e.g. when VSYSCALL emulation is disabled.

So increase the static space to 4M for now by defining FIXMAP_PMD_NUM to 2,
and add a build time check to ensure that the fixmap is covered by the
initial static page tables.

Fixes: 1ad83c858c7d ("x86_64,vsyscall: Make vsyscall emulation configurable")
Suggested-by: Thomas Gleixner <[email protected]>
Signed-off-by: Feng Tang <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Tested-by: kernel test robot <[email protected]>
Reviewed-by: Juergen Gross <[email protected]> (Xen parts)
Cc: H Peter Anvin <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Yinghai Lu <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Andy Lutomirsky <[email protected]>
Cc: [email protected]
Link: https://lkml.kernel.org/r/[email protected]

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