ocfs2: Limit default local alloc size within bitmap range.

In commit 6b82021b

, we increase
our local alloc size and calculate how much megabytes we can
get according to group size and volume size.
But we also need to check the maximum bits a local alloc block
bitmap can have. With a bs=512, cs=32K, local volume with 160G,
it calculate 96MB while the maximum local alloc size is only
76M. So the bitmap will overflow and corrupt the system truncate
log file. See bug
http://oss.oracle.com/bugzilla/show_bug.cgi?id=1262

Signed-off-by: Tao Ma <tao.ma@oracle.com>
Acked-by: Mark Fasheh <mfasheh@suse.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>