dm vdo: use a short static string for thread name prefix

Also remove MODULE_NAME and a BUG_ON check, both unneeded.

This fixes a warning about string truncation in snprintf that
will never happen in practice:

drivers/md/dm-vdo/vdo.c: In function ‘vdo_make’:
drivers/md/dm-vdo/vdo.c:564:5: error: ‘%s’ directive output may be truncated writing up to 55 bytes into a region of size 16 [-Werror=format-truncation=]
    "%s%u", MODULE_NAME, instance);
     ^~
drivers/md/dm-vdo/vdo.c:563:2: note: ‘snprintf’ output between 2 and 66 bytes into a destination of size 16
  snprintf(vdo->thread_name_prefix, sizeof(vdo->thread_name_prefix),
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    "%s%u", MODULE_NAME, instance);
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Reported-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Signed-off-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>