erofs: suppress false positive last_block warning

As Andrew mentioned, some rare specific gcc versions could report
last_block uninitialized warning. Actually last_block doesn't need
to be uninitialized first from its implementation due to bio == NULL
condition. After a bio is allocated, last_block will be assigned
then.

The detailed analysis is in this thread [1]. So let's silence those
confusing gccs simply.

[1] https://lore.kernel.org/r/20200421072839.GA13867@hsiangkao-HP-ZHAN-66-Pro-G1



Cc: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Link: https://lore.kernel.org/r/20200528084844.23359-1-hsiangkao@redhat.com


Signed-off-by: Gao Xiang <hsiangkao@redhat.com>