gfs2: Only use struct gfs2_rbm for bitmap manipulations

GFS2 uses struct gfs2_rbm to represent a filesystem block number as a
bit position within a resource group.  This representation is used in
the bitmap manipulation code to prevent excessive conversions between
block numbers and bit positions, but also in struct gfs2_blkreserv which
is part of struct gfs2_inode, to mark the start of a reservation.  In
the inode, the bit position representation makes less sense: first, the
start position is used as a block number about as often as a bit
position; second, the bit position representation makes the code
unnecessarily complicated and difficult to read.

Therefore, change struct gfs2_blkreserv to represent the start of a
reservation as a block number instead of a bit position.  (This requires
keeping track of the resource group in gfs2_blkreserv separately.) With
that change, various things can be slightly simplified, and struct
gfs2_rbm can be moved to rgrp.c.

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