mke2fs: warn if not enabling all the features that metadata_csum wants

mke2fs: warn if not enabling all the features that metadata_csum wants

The metadata_csum feature works best when two features are enabled.
These features are "extents" (because the block map has no space for
checksums) and "64bit" (this enables storage of full 32-bit checksums
in certain fields).  Print a warning if the user tries to create a
filesystem without those features.

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>