device: if BLKPBSZGET is unavailable, enforce 512

If there is no define for BLKPBSZGET - we have hard time how to
decrypt physical block size - we can't use here block_size,
since this is usually 4k while we need to use 512b.

FIXME: find some better way, until that enforce value 512.
Eventually we could also try to put in:

+#ifndef BLKPBSZGET
+# define BLKPBSZGET _IO(0x12,123)
+#endif

but this will still not work well on old kernels.

lib/device/dev-io.c

@@ -157,7 +157,8 @@ int dev_get_block_size(struct device *dev, unsigned int *physical_block_size, un
 	}
 #else
 	/* if we can't get physical block size, just use logical block size instead */
-	dev->phys_block_size = dev->block_size;
+	// FIXME block_size is typically 4096b while phys_block_size is 512b
+	dev->phys_block_size = 512;// dev->block_size;
 	log_debug_devs("%s: physical block size can't be determined, using logical "
 		       "block size of %u bytes instead", name, dev->phys_block_size);
 #endif