fs: out of bounds on stack in iov_iter_advance

On Wed, Nov 11, 2015 at 10:19:48AM +0000, Al Viro wrote:

> I'll cook the minimal fixup for API change after I get some sleep and
> send it your way, unless somebody gets there first...

This should do it - switches ->ioctl() to pvfs2_inode_[gs]etxattr() and
converts xattr_handler ->[gs]et() to new API.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Mike Marshall <hubcap@omnibond.com>