staging: erofs: introduce erofs_grab_bio

this patch renames prepare_bio to erofs_grab_bio, and
adds a nofail option in order to retry in the bio allocator
under memory pressure.

Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>