fs/ext4: fix log2blksz un-initialized error, by cacaulating its value from blksz

The problem here is that uboot can't mount ext4 filesystem with
commit "50ce4c07df1" applied. We use hard-coded "SECTOR_SIZE"(512)
before this commit, now we introduce (block_dev_desc_t *)->log2blksz
to replace this macro. And after we calling do_ls()->fs_set_blk_dev(),
the variable log2blksz is not initialized, which it's not correct.

And this patch try to solve the problem by caculating the value of
log2blksz from variable blksz.

disk/part.c

@@ -548,6 +548,8 @@ int get_device_and_partition(const char *ifname, const char *dev_part_str,
 			goto cleanup;
 		}
 
+		(*dev_desc)->log2blksz = LOG2((*dev_desc)->blksz);
+
 		info->start = 0;
 		info->size = (*dev_desc)->lba;
 		info->blksz = (*dev_desc)->blksz;
@@ -631,6 +633,8 @@ int get_device_and_partition(const char *ifname, const char *dev_part_str,
 		goto cleanup;
 	}
 
+	(*dev_desc)->log2blksz = LOG2((*dev_desc)->blksz);
+
 	ret = part;
 	goto cleanup;