ovl: Copy up underlying inode's ->i_mode to overlay inode

ovl: Copy up underlying inode's ->i_mode to overlay inode

Right now when a new overlay inode is created, we initialize overlay
inode's ->i_mode from underlying inode ->i_mode but we retain only
file type bits (S_IFMT) and discard permission bits.

This patch changes it and retains permission bits too. This should allow
overlay to do permission checks on overlay inode itself in task context.

[SzM] It also fixes clearing suid/sgid bits on write.

Signed-off-by: Vivek Goyal <[email protected]>
Reported-by: Eryu Guan <[email protected]>
Signed-off-by: Miklos Szeredi <[email protected]>
Fixes: 4bacc9c9234c ("overlayfs: Make f_path always point to the overlay and f_inode to the underlay")
Cc: <[email protected]>