packet: fix error handling

The packet driver is assuming (reasonably) that the (undocumented)
request.errors is an errno.  But it is in fact some mysterious bitfield.  When
things go wrong we return weird positive numbers to the VFS as pointers and it
goes oops.

Thanks to William Heimbigner for reporting and diagnosis.

(It doesn't oops, but this driver still doesn't work for William)

Cc: William Heimbigner <icxcnika@mar.tar.cc>
Cc: Peter Osterlund <petero2@telia.com>
Cc: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>