ubifs: Correct the total block count by deducting journal reservation

[ Upstream commit 84a2bee9c49769310efa19601157ef50a1df1267 ]

Since commit e874dcde1cbf ("ubifs: Reserve one leb for each journal
head while doing budget"), available space is calulated by deducting
reservation for all journal heads. However, the total block count (
which is only used by statfs) is not updated yet, which will cause
the wrong displaying for used space(total - available).
Fix it by deducting reservation for all journal heads from total
block count.

Fixes: e874dcde1cbf ("ubifs: Reserve one leb for each journal head while doing budget")
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Sasha Levin <sashal@kernel.org>