jfs: UBSAN: shift-out-of-bounds in dbFindBits

[ Upstream commit b0b2fc815e514221f01384f39fbfbff65d897e1c ]

Fix issue with UBSAN throwing shift-out-of-bounds warning.

Reported-by:  <syzbot+e38d703eeb410b17b473@syzkaller.appspotmail.com>
Signed-off-by: Remington Brasga <rbrasga@uci.edu>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>