mkswap: various minor improvement

mkswap: various minor improvement

Use correct data type in page_bad(), and add information to error message
how many bad pages were seen.

In check_blocks() move initialization to variable introduction, fix typo,
and avoid memset() when array initializer can do the job.

In main() use correct initializer for pointer.  Move swap file specific
actions to same location, and add warning when request too --check is
dismissed by command.  At the end remove momory leak.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>