f2fs: fix the wrong condition to determine atomic context

Should use !in_task for irq context.

Cc: stable@vger.kernel.org
Fixes: 1aa161e4 ("f2fs: fix scheduling while atomic in decompression path")
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>