FROMLIST: seq_file: add seq_read_iter

iov_iter based variant for reading a seq_file.  seq_read is
reimplemented on top of the iter variant.

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: I216f87fbe072a1e3a41c597df574c43d6d21ba80