CVE-2026-64294: mm: do file ownership checks with the proper mount idmap

In the Linux kernel, the following vulnerability has been resolved:

mm: do file ownership checks with the proper mount idmap

Ever since idmapped mounts were introduced, inode ownership checks (for side-channel protection) in mincore() and madvise(MADVPAGEOUT) were done against the nopmntidmap, which completely ignores the file's mount's idmap. This results in odd edgecases like:

1) mount/bind-mount with an idmap userA:userB:1 2) userB runs an ownerorcapable() check on file that is owned by userA on-disk/in-memory, but owned by userB after idmap translation 3) ownerorcapable() mysteriously fails as the correct idmap wasn't supplied

In the case of mincore/madvise MADVPAGEOUT, this is usually benign, because filepermission(file, MAYWRITE) will probably succeed, as it uses the proper idmap internally, but it does not need to be the case on e.g a 0444 file where even the owner itself doesn't have permissions to write to it.

Since this is clearly not trivial to get right, introduce a fileownerorcapable() that can carry the correct semantics, and switch the various users in mm to it.

The issue was found by manual code inspection & an off-list discussion with Jan Kara.