9p: switch p9_client_write() to passing it struct iov_iter *

... and make it loop until it's done

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --cc fs/9p/vfs_file.c
Simple merge

diff --cc fs/9p/xattr.c
Simple merge

diff --cc include/net/9p/client.h
Simple merge

diff --cc net/9p/client.c
Simple merge