UPSTREAM: 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>

Bug: 297961372
(cherry picked from commit 9f2731d6)
Change-Id: I9331cb53a7a8ff0ae7bc00c2ef09cc70715cf162
Signed-off-by: Edward Wu <edwardwu@realtek.com>