gfs2: Even more gfs2_find_jhead fixes

Fix several issues in the previous gfs2_find_jhead fix:
* When updating @blocks_submitted, @block refers to the first block block not
  submitted yet, not the last block submitted, so fix an off-by-one error.
* We want to ensure that @blocks_submitted is far enough ahead of @blocks_read
  to guarantee that there is in-flight I/O.  Otherwise, we'll eventually end up
  waiting for pages that haven't been submitted, yet.
* It's much easier to compare the number of blocks added with the number of
  blocks submitted to limit the maximum bio size.
* Even with bio chaining, we can keep adding blocks until we reach the maximum
  bio size, as long as we stop at a page boundary.  This simplifies the logic.

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