define GENERIC_IOMAP in a central location
instead of all architectures. This will be helpful
for the follow-up patch which makes it select
other configs. Code is also a bit shorter this way.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

Signed-off-by: NLinas Vepstas <linas@codeaurora.org>
Signed-off-by: NRichard Kuo <rkuo@codeaurora.org>
Acked-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>