Update the xattrs to use the item cache. Because we now have large keys
we can store the xattr at its full name instead of having to deal with
hashing the name and addressing collisions.
Now that we don't have the find xattr ioctls we don't need to maintain
backrefs.
We also add support for large xattrs that span multiple items. The key
footer and value header give us the metadata we need to iterate over the
items that make up an xattr.
Signed-off-by: Zach Brown <zab@versity.com>