erofs: force inplace I/O under low memory scenario

Try to forcely switch to inplace I/O under low memory scenario in
order to avoid direct memory reclaim due to cached page allocation.

Link: https://lore.kernel.org/r/20201209123717.12430-1-hsiangkao@aol.com


Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>