fs/jfs: fix shift exponent db_agl2size negative

As a shift exponent, db_agl2size can not be less than 0. Add the missing
check to fix the shift-out-of-bounds bug reported by syzkaller:

 UBSAN: shift-out-of-bounds in fs/jfs/jfs_dmap.c:2227:15
 shift exponent -744642816 is negative

Reported-by:  <syzbot+0be96567042453c0c820@syzkaller.appspotmail.com>
Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>