iov_iter based variant for reading a seq_file.  seq_read is
reimplemented on top of the iter variant.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Tested-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>