tests: test "mke2fs -E offset=N" filesystem size calculation

tests: test "mke2fs -E offset=N" filesystem size calculation

If "mke2fs -E offset=N ..." is called without a specified filesystem size,
the offset should be subtracted from the default filesystem size (see
commit 87d9b2fb). If "mke2fs -E offset=N ..." is called with an
explicitly specified filesystem size, the specified filesystem size
should be used.

Also, call dd with the "iflag=fullblock" option, if it is going to read
from a pipe.

Signed-off-by: Marcus Huewe <suse-tux@gmx.de>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>