[GFS2] Make the log reserved blocks depend on block size

The number of blocks which we reserve in the log at the start of each
transaction needs to depends upon the block size since the overhead is
related to the number of "pointers" which can be fitted into a single
block.

This relates to Red Hat bz #240435

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

fs/gfs2/log.c

@@ -262,7 +262,7 @@ static void ail2_empty(struct gfs2_sbd *sdp, unsigned int new_tail)
  * @sdp: The GFS2 superblock
  * @blks: The number of blocks to reserve
  *
- * Note that we never give out the last 6 blocks of the journal. Thats
+ * Note that we never give out the last few blocks of the journal. Thats
  * due to the fact that there is are a small number of header blocks
  * associated with each log flush. The exact number can't be known until
  * flush time, so we ensure that we have just enough free blocks at all
@@ -274,14 +274,15 @@ static void ail2_empty(struct gfs2_sbd *sdp, unsigned int new_tail)
 int gfs2_log_reserve(struct gfs2_sbd *sdp, unsigned int blks)
 {
 	unsigned int try = 0;
+	unsigned reserved_blks = 6 * (4096 / sdp->sd_vfs->s_blocksize);
 
 	if (gfs2_assert_warn(sdp, blks) ||
 	    gfs2_assert_warn(sdp, blks <= sdp->sd_jdesc->jd_blocks))
 		return -EINVAL;
 
 	mutex_lock(&sdp->sd_log_reserve_mutex);
 	gfs2_log_lock(sdp);
-	while(sdp->sd_log_blks_free <= (blks + 6)) {
+	while(sdp->sd_log_blks_free <= (blks + reserved_blks)) {
 		gfs2_log_unlock(sdp);
 		gfs2_ail1_empty(sdp, 0);
 		gfs2_log_flush(sdp, NULL);