Relax the rw_verify_area() error checking.

In particular, allow over-large read- or write-requests to be downgraded
to a more reasonable range, rather than considering them outright errors.

We want to protect lower layers from (the sadly all too common) overflow
conditions, but prefer to do so by chopping the requests up, rather than
just refusing them outright.

Cc: Peter Anvin <hpa@zytor.com>
Cc: Ulrich Drepper <drepper@redhat.com>
Cc: Andi Kleen <ak@suse.de>
Cc: Al Viro <viro@ftp.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>