x86/mm: Tabulate the page table encoding definitions

I got lost in trying to figure out which bits were enabled
in one of the PTE masks, so let's make it pretty
obvious at the definition site already:

 #define PAGE_NONE            __pg(   0|   0|   0|___A|   0|   0|   0|___G)
 #define PAGE_SHARED          __pg(__PP|__RW|_USR|___A|__NX|   0|   0|   0)
 #define PAGE_SHARED_EXEC     __pg(__PP|__RW|_USR|___A|   0|   0|   0|   0)
 #define PAGE_COPY_NOEXEC     __pg(__PP|   0|_USR|___A|__NX|   0|   0|   0)
 #define PAGE_COPY_EXEC       __pg(__PP|   0|_USR|___A|   0|   0|   0|   0)
 #define PAGE_COPY            __pg(__PP|   0|_USR|___A|__NX|   0|   0|   0)
 #define PAGE_READONLY        __pg(__PP|   0|_USR|___A|__NX|   0|   0|   0)
 #define PAGE_READONLY_EXEC   __pg(__PP|   0|_USR|___A|   0|   0|   0|   0)

 #define __PAGE_KERNEL            (__PP|__RW|   0|___A|__NX|___D|   0|___G)
 #define __PAGE_KERNEL_EXEC       (__PP|__RW|   0|___A|   0|___D|   0|___G)
 #define _KERNPG_TABLE_NOENC      (__PP|__RW|   0|___A|   0|___D|   0|   0)
 #define _KERNPG_TABLE            (__PP|__RW|   0|___A|   0|___D|   0|   0| _ENC)
 #define _PAGE_TABLE_NOENC        (__PP|__RW|_USR|___A|   0|___D|   0|   0)
 #define _PAGE_TABLE              (__PP|__RW|_USR|___A|   0|___D|   0|   0| _ENC)
 #define __PAGE_KERNEL_RO         (__PP|   0|   0|___A|__NX|___D|   0|___G)
 #define __PAGE_KERNEL_RX         (__PP|   0|   0|___A|   0|___D|   0|___G)
 #define __PAGE_KERNEL_NOCACHE    (__PP|__RW|   0|___A|__NX|___D|   0|___G| __NC)
 #define __PAGE_KERNEL_VVAR       (__PP|   0|_USR|___A|__NX|___D|   0|___G)
 #define __PAGE_KERNEL_LARGE      (__PP|__RW|   0|___A|__NX|___D|_PSE|___G)
 #define __PAGE_KERNEL_LARGE_EXEC (__PP|__RW|   0|___A|   0|___D|_PSE|___G)
 #define __PAGE_KERNEL_WP         (__PP|__RW|   0|___A|__NX|___D|   0|___G| __WP)

Especially security relevant bits like 'NX' or coherence related bits like 'G'
are now super easy to read based on a single grep.

We do the underscore gymnastics to not pollute the kernel's symbol namespace,
and the longest line still fits into 80 columns, so this should be readable
for everyone.

Signed-off-by: Ingo Molnar <mingo@kernel.org>