Make ioctl.h compatible with userland

The attached patch seems to already exist in a number of branches -- it
keeps popping up on Google for me, and is certainly already in Debian --
but is strangely absent from mainstream.

The problem appears to be that the patched file ends up as part of the
target toolchain, but unfortunately the gcc constant folding doesn't
appear to eliminate the __invalid_size_argument_for_IOC value early
enough.  Certainly compiling C++ programs which use _IO...  macros as
constants fails without this patch.

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