f2fs: mark f2fs_init_compress_mempool w/ __init

f2fs_init_compress_mempool() only initializes the memory pool during
the f2fs module init phase. Let's mark it as __init like any other
function.

Signed-off-by: Yangtao Li <frank.li@vivo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>