compat: factor out compat_rw_copy_check_uvector from compat_do_readv_writev

It was reported in http://lkml.org/lkml/2010/3/8/309

 that 32 bit readv and
writev AIO operations were not functioning properly.  It turns out that
the code to convert the 32bit io vectors to 64 bits was never written.
The results of that can be pretty bad, but in my testing, it mostly ended
up in generating EFAULT as we walked off the list of I/O vectors provided.

This patch set fixes the problem in my environment.  are greatly
appreciated.

This patch:

Factor out code that will be used by both compat_do_readv_writev and the
compat aio submission code paths.

Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Reported-by: Michael Tokarev <mjt@tls.msk.ru>
Cc: Zach Brown <zach.brown@oracle.com>
Cc: <stable@kernel.org>		[2.6.35.1]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>