staging: erofs: use the new LZ4_decompress_safe_partial()

LZ4_decompress_safe_partial() can now decode exactly the nb of bytes
requested after the upstreamed commit 2209fda3 ("lib/lz4: update LZ4
decompressor module"), remove the erofs hacked lz4 decompression code.

A more cleaned compressor wrapper will be introduced in the subsequent
patches in order to prepare for supporting multiple compression algorithms.

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