fs/jfs: fix shift exponent db_agl2size negative

stable inclusion
from stable-v5.10.173
commit 5d03a19ac7e81f80d96c3bfeb719da38257f75e5
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I7X0QU

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5d03a19ac7e81f80d96c3bfeb719da38257f75e5



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[ Upstream commit fad376fc ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: sanglipeng <sanglipeng1@jd.com>