jfs: validate max amount of blocks before allocation.

The lack of checking bmp->db_max_freebud in extBalloc() can lead to
shift out of bounds, so this patch prevents undefined behavior, because
bmp->db_max_freebud == -1 only if there is no free space.

Signed-off-by: Aleksei Filippov <halip0503@gmail.com>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Fixes: 1da177e4

 ("Linux-2.6.12-rc2")
Reported-and-tested-by:  <syzbot+5f088f29593e6b4c8db8@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?id=01abadbd6ae6a08b1f1987aa61554c6b3ac19ff2