LU-11609 mke2fs: allow 64bit feature without extents

LU-11609 mke2fs: allow 64bit feature without extents

The 64bit feature should be allowed without extents to for 32-bit
metadata_csum checksums to be stored in the group descriptor.
Change the extents check to check for more than 2^32 blocks instead
of the 64bit feature flag.  This also avoids warnings later if the
metadata_csum feature is enabled on a filesystem without 64bit.

Consolidate repeated 64bit feature checks into one check.

The fs_blocks_count value is always set when parse_fs_type() is
called, so no need to check if it is zero.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ie73ff0ed50cfed5d6a9596260c6b6dc32d3ebbe5
Reviewed-on: https://review.whamcloud.com/33897
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Li Dongyang <dongyangli@ddn.com>