mke2fs: Fix up the fs type and feature selection for 64-bit blocks

mke2fs: Fix up the fs type and feature selection for 64-bit blocks

We need to defer setting the blocks count field in the fs_param
structure until it is known whether 64-bit feature will be enabled
(and whether the blocks count is valid).

We also add a new mke2fs.conf configuration parameter,
auto_64-bit_support which will automatically enable the 64-bit feature
if the number of blocks requires it.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>