x86/mm: Identify the end of the kernel area to be reserved

The memory occupied by the kernel is reserved using memblock_reserve()
in setup_arch(). Currently, the area is from symbols _text to __bss_stop.
Everything after __bss_stop must be specifically reserved otherwise it
is discarded. This is not clearly documented.

Add a new symbol, __end_of_kernel_reserve, that more readily identifies
what is reserved, along with comments that indicate what is reserved,
what is discarded and what needs to be done to prevent a section from
being discarded.

Signed-off-by: Tom Lendacky <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Reviewed-by: Baoquan He <[email protected]>
Reviewed-by: Dave Hansen <[email protected]>
Tested-by: Lianbo Jiang <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Brijesh Singh <[email protected]>
Cc: Dave Young <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Joerg Roedel <[email protected]>
Cc: Juergen Gross <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Nick Desaulniers <[email protected]>
Cc: Pavel Tatashin <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Robert Richter <[email protected]>
Cc: Sami Tolvanen <[email protected]>
Cc: Sinan Kaya <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: "[email protected]" <[email protected]>
Link: https://lkml.kernel.org/r/7db7da45b435f8477f25e66f292631ff766a844c.1560969363.git.thomas.lendacky@amd.com

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