proc: handle subset=pid separately in userns visibility checks

proc: handle subset=pid separately in userns visibility checks

When procfs is mounted with subset=pid, only the dynamic process-related
part of the filesystem remains visible. That part cannot be hidden by
overmounts, so checking whether an existing procfs mount is fully
visible does not make sense for this mode.

At the same time, a subset=pid procfs mount must not be used as evidence
that a later procfs mount would not reveal additional information. It
provides a restricted view of procfs, not the full filesystem view.

Mark subset=pid procfs instances as restricted variants. Ignore
restricted variants when looking for an already-visible mount, and allow
new restricted variants without consulting mnt_already_visible().

Signed-off-by: Alexey Gladkov <legion@kernel.org>
Link: https://patch.msgid.link/4d5e760c3d534dd2e05578d119cc408450053a98.1777278334.git.legion@kernel.org
Reviewed-by: Aleksa Sarai <aleksa@amutable.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>