gfs2: Add per-reservation reserved block accounting

Add a rs_reserved field to struct gfs2_blkreserv to keep track of the number of
blocks reserved by this particular reservation, and a rd_reserved field to
struct gfs2_rgrpd to keep track of the total number of reserved blocks in the
resource group.  Those blocks are exclusively reserved, as opposed to the
rs_requested / rd_requested blocks which are tracked in the reservation tree
(rd_rstree) and which can be stolen if necessary.

When making a reservation with gfs2_inplace_reserve, rs_reserved is set to
somewhere between ap->min_target and ap->target depending on the number of free
blocks in the resource group.  When allocating blocks with gfs2_alloc_blocks,
rs_reserved is decremented accordingly.  Eventually, any reserved but not
consumed blocks are returned to the resource group by gfs2_inplace_release.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>