mm: Reserve asm-generic prot flags 0x10 and 0x20 for arch use

mm: Reserve asm-generic prot flags 0x10 and 0x20 for arch use

The asm-generic/mman.h definitions are used by a few architectures that
also define arch-specific PROT flags with value 0x10 and 0x20. This
currently applies to sparc and powerpc for 0x10, while arm64 will soon
join with 0x10 and 0x20.

To help future maintainers, document the use of this flag in the
asm-generic header too.

Acked-by: Arnd Bergmann <[email protected]>
Signed-off-by: Dave Martin <[email protected]>
[[email protected]: reserve 0x20 as well]
Signed-off-by: Catalin Marinas <[email protected]>
Signed-off-by: Will Deacon <[email protected]>