hfsplus: remove can_set_xattr

When using the per-superblock xattr handlers permission checking is
done by the generic code.  hfsplus just needs to check for the magic
osx attribute not to leak into protected namespaces.

Also given that the code was obviously copied from JFS the proper
attribution was missing.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>