design: fix computation of buffer log item bitmap size

design: fix computation of buffer log item bitmap size

Wengang Wang was trying to work through a buffer log item by consulting
the ondisk format documentation, and was confused by the formula given
in section 14.3.14 regarding the size of blf_data_map, aka the dirty
bitmap for buffer log items.  We noticed that the documentation doesn't
match what the kernel actually does, so let's fix this.

Reported-by: Wengang Wang <wen.gang.wang@oracle.com>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Chandan Babu R <chandan.babu@oracle.com>