gfs2: Remove unused RGRP_RSRV_MINBYTES definition

This definition is only used to define RGRP_RSRV_MINBLKS, with no
benefit over defining RGRP_RSRV_MINBLKS directly.

In addition, instead of forcing RGRP_RSRV_MINBLKS to be of type u32,
cast it to that type where that type is required.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Reviewed-by: Steven Whitehouse <swhiteho@redhat.com>