libnvdimm, label: change min label storage size per UEFI 2.7

UEFI 2.7 defines in page 758 that:

  Initial Label Storage Area Configuration
     :
  The minimum size of the Label Storage Area is large enough to
  hold 2 index blocks and 2 labels.

The mininum index block size is 256 bytes, and the minimum label size
is also 256 bytes.

Change ND_LABEL_MIN_SIZE to (256 * 4) so that NVDIMM devices with
the minimum label storage area do not fail with the size check in
nvdimm_init_config_data().
Signed-off-by: NToshi Kani <toshi.kani@hpe.com>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>

drivers/nvdimm/label.h

@@ -33,7 +33,7 @@ enum {
 	BTTINFO_UUID_LEN = 16,
 	BTTINFO_FLAG_ERROR = 0x1,    /* error state (read-only) */
 	BTTINFO_MAJOR_VERSION = 1,
-	ND_LABEL_MIN_SIZE = 512 * 129, /* see sizeof_namespace_index() */
+	ND_LABEL_MIN_SIZE = 256 * 4, /* see sizeof_namespace_index() */
 	ND_LABEL_ID_SIZE = 50,
 	ND_NSINDEX_INIT = 0x1,
 };