mke2fs: fix hugefile creation so the hugefile(s) are contiguous

Commit 4f868703f6f2: "libext2fs: use fallocate for creating journals
and hugefiles" introduced a regression for the mke2fs hugefile
feature.  The problem is that the fallocate library function
intersperses the extent tree metadata blocks with the data blocks, and
this violates the hugefile guarantee that the created files should be
physically contiguous on disk.

Unfortuantely the m_hugefile regression test was flawed, and didn't
pick up the regression.

This commit fixes the regression test so that it detects the problem
before fixing mke2fs, and also fixes the mke2fs hugefile by reverting
the mke2fs hugefile portion of commit 4f868703f6f2.

Google-Bug-Id: 62791459

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