Implemented tst_fill_file function

Signed-off-by: Stanislav Kholmanskikh <stanislav.kholmanskikh@oracle.com>
Reviewed-by: Cyril Hrubis <chrubis@suse.cz>
syscalls/swapon: fix for variable page size

mkswap refuses files of size < 10*(page size).

On systems with 8192 page size swapon02, swapon03 tests fail because
they try to use files of size 41920 (< 81920).

Modified swapon1, swapon02, swapon03 so they operate on files of size
10*(page size).

Verified on systems with 4096 and 8192 page sizes.

Signed-off-by: Stanislav Kholmanskikh <stanislav.kholmanskikh@oracle.com>
Reviewed-by: Cyril Hrubis <chrubis@suse.cz>