This patch converts blk_t type to __u64 when compiling code with 
"_EXT4FS_" option.
Force the 64-bit feature when the filesystem is greater than 2**32 blocks.

Changes from the previous version:
- ask for confirmation before creating the ext4 filesystem

  lib/ext2fs/ext2_fs.h |    3 ++-
  lib/ext2fs/ext2fs.h  |   15 +++++++++++----
  misc/mke2fs.c        |   14 +++++++++++++-
  3 files changed, 26 insertions(+), 6 deletions(-)