fs: add vfs_iter_{read,write} helpers

Simple helpers that pass an arbitrary iov_iter to filesystems.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --cc fs/read_write.c
Simple merge

diff --cc fs/splice.c
Simple merge

diff --cc include/linux/fs.h
Simple merge