erofs: fold in should_decompress_synchronously()

should_decompress_synchronously() has one single condition
for now, so fold it instead.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Link: https://lore.kernel.org/r/20200919072730.24989-2-hsiangkao@redhat.com


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