pidfd: add NSpid entries to fdinfo

Currently, the fdinfo file contains the Pid field which shows the
pid a given pidfd refers to in the pid namespace of the procfs
instance. If pid namespaces are configured, also show an NSpid field
for easy retrieval of the pid in all descendant pid namespaces. If
the pid namespace of the process is not a descendant of the pid
namespace of the procfs instance 0 will be shown as its first NSpid
entry and no other entries will be shown. Add a block comment to
pidfd_show_fdinfo with a detailed explanation of Pid and NSpid fields.

Co-developed-by: Christian Brauner <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>
Signed-off-by: Christian Kellner <[email protected]>
Reviewed-by: Christian Brauner <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Christian Brauner <[email protected]>

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