fs/xattr.c:listxattr(): fall back to vmalloc() if kmalloc() failed

fs/xattr.c:listxattr(): fall back to vmalloc() if kmalloc() failed

This allocation can be as large as 64k.  As David points out, "falling
back to vmalloc here is much better solution than failing to retreive the
attribute - it will work no matter how fragmented memory gets.  That means
we don't get incomplete backups occurring after days or months of uptime
and successful backups".

Cc: Dave Chinner <david@fromorbit.com>
Cc: Dave Jones <davej@codemonkey.org.uk>
Cc: David Rientjes <rientjes@google.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>