coredump: use 'input_fd' name for the pipe fd passed by the kernel everywhere

coredump: use 'input_fd' name for the pipe fd passed by the kernel everywhere

'input_fd' variable name is used mostly everywhere except in process_socket()
where it's named 'coredump_fd', which is pretty confusing since 'coredump_fd'
is used for the coredump filename in submit_coredump().

So let's use 'input_fd' consistently as name for the pipe fd passed by the
kernel.

No functional changes.