f2fs: fix wrong error injection path in inc_valid_block_count()

If FAULT_BLOCK type error injection is on, in inc_valid_block_count()
we may decrease sbi->alloc_valid_block_count percpu stat count
incorrectly, fix it.

Fixes: 36b877af ("f2fs: Keep alloc_valid_block_count in sync")
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>