ioremap: Delay sanity check until after a successful mapping

While tracking down the reason for an ioremap() failure I was
distracted  by the WARN_ONCE() in __ioremap_caller().

Performing a WARN_ONCE() sanity check before the mapping
is successful seems pointless if the caller sends bad values.

A case in point is when the BIOS provides erroneous screen_info
values causing vesafb_probe() to request an outrageuous size.
The WARN_ONCE is then wasted on bogosity. Move the warning to a
point where the mapping has been successfully allocated.

Addresses:

  http://bugs.launchpad.net/bugs/772042

Reviewed-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Link: http://lkml.kernel.org/r/4DB99D2E.9080106@canonical.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>