mke2fs: adjust the default file system size by the offset

mke2fs: adjust the default file system size by the offset

If the user specifies an offset using -E offset without specifying an
explicit size, the system will use the block device (or file) size as
the default file system size.  If we are using the default file system
size, subtract out the offset so the resulting file system will stay
within the block device.  Also print a warning that this might not be
what the user wants.

Addresses-Debian-Bug: #803629

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