asm-generic/cacheflush.h: flush icache when copying to user pages

The copy_to_user_page() function is supposed to flush the icache on the
memory that was written, but the current asm-generic version lacks that
logic.  While normally it isn't a big deal as the asm-generic version of
icache flushing is a stub, it is a deal for ports that want to use the
asm-generic version as a baseline and then overlay its own specific parts
(like icache flushing).

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>