gfs2: Convert function bh_get to use iomap

Before this patch, function bh_get used block_map to figure out the
block it needed to read in from the quota_change file. This patch
changes it to use iomap directly to make it more efficient.

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