f2fs: simplify code in f2fs_prepare_decomp_mem

It could return directly after init_decompress_ctx.

Signed-off-by: Zhang Qilong <zhangqilong3@huawei.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>