FROMLIST: proc: wire up generic_file_splice_read for iter ops

Wire up generic_file_splice_read for the iter based proxy ops, so
that splice reads from them work.

Bug: 171770067
Link: https://lore.kernel.org/r/20201027080745.GA31045@infradead.org


Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I163a0609fd23059121d0f28359abe2a4b39b5031