erofs: avoid unnecessary z_erofs_decompressqueue_work() declaration

Just code rearrange. No logic changes.

Link: https://lore.kernel.org/r/20220121091412.86086-1-hsiangkao@linux.alibaba.com


Reviewed-by: Yue Hu <huyue2@yulong.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>