erofs: refine managed inode stuffs

Set up the correct gfp mask and use it instead of hard coding.
Also add comments about .invalidatepage() to show more details.

Link: https://lore.kernel.org/r/20220310182743.102365-2-hsiangkao@linux.alibaba.com


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