asm-generic: improve the flush_dcache_page stub

There is a magic ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE cpp symbol that
guards non-stub availability of flush_dcache_pagge.  Use that to check
if flush_dcache_pagg is implemented.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Link: http://lkml.kernel.org/r/20200515143646.3857579-8-hch@lst.de


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>