jfs: check if leafidx greater than num leaves per dmap tree

syzbot report a out of bounds in dbSplit, it because dmt_leafidx greater
than num leaves per dmap tree, add a checking for dmt_leafidx in dbFindLeaf.

Shaggy:
Modified sanity check to apply to control pages as well as leaf pages.

Reported-and-tested-by:  <syzbot+dca05492eff41f604890@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=dca05492eff41f604890


Signed-off-by: Edward Adam Davis <eadavis@qq.com>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>