mkswap: unbreak -c ("check") option.

mkswap: unbreak -c ("check") option.

In the check_blocks() loop, current_page is not incremented. Because of this
bug, the loop does not end when the end of the device is reached.  Instead it
tries to continue reading, which of course fails, and eventually mkswap aborts
("too many bad pages").

Note that this can only be tested with swap partitions, mkswap ignores the -c
option for swap files.

Signed-off-by: Peter De Wachter <pdewacht@gmail.com>