GFS2: optimize rbm_from_block wrt bi_start

In function gfs2_rbm_from_block, it starts by checking if the block
falls within the first bitmap. It does so by checking if the rbm's
offset is less than (rbm->bi->bi_start + rbm->bi->bi_len) * GFS2_NBBY.
However, the first bitmap will always have bi_start==0. Therefore
this is an unnecessary calculation in a function that gets called
billions of times. This patch removes the reference to bi_start.
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

fs/gfs2/rgrp.c

@@ -262,7 +262,7 @@ static int gfs2_rbm_from_block(struct gfs2_rbm *rbm, u64 block)
 	rbm->bi = rbm->rgd->rd_bits;
 	rbm->offset = (u32)(rblock);
 	/* Check if the block is within the first block */
-	if (rbm->offset < (rbm->bi->bi_start + rbm->bi->bi_len) * GFS2_NBBY)
+	if (rbm->offset < rbm->bi->bi_len * GFS2_NBBY)
 		return 0;
 
 	/* Adjust for the size diff between gfs2_meta_header and gfs2_rgrp */