net, pidfd: report EINVAL for ESRCH

dbus-broker relies -EINVAL being returned to indicate ESRCH in [1].
This causes issues for some workloads as reported in [2].
Paper over it until this is fixed in userspace.

Link: https://lore.kernel.org/20250416-gegriffen-tiefbau-70cfecb80ac8@brauner
Link: https://github.com/bus1/dbus-broker/blob/5d34d91b138fc802a016aa68c093eb81ea31139c/src/util/sockopt.c#L241 [1]
Link: https://lore.kernel.org/20250415223454.GA1852104@ax162

 [2]
Tested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>