Handle arbitrary base and length values as long as they
are multiples of IO_PAGE_SIZE.

Bug found by Arun Kumar Rao.
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>