mke2fs: do not warn about a pre-existing partition table when using a non-zero offset

mke2fs: do not warn about a pre-existing partition table when using a non-zero offset

The existing code attempted to avoid warning about a pre-existing file
system with a non-zero offset, but because the offset was not set at
the time of the check, this intention was not actually working.  So
this commit will suppress warnings about pre-existing a partition
table as well as pre-existing file system when there is a non-zero
offset.

Addresses-Debian-Bug: #989612
Signed-off-by: Theodore Ts'o <tytso@mit.edu>