rxrpc: make the users of rxrpc_kernel_send_data() set kvec-backed msg_iter properly

Use iov_iter_kvec() there, get rid of set_fs() games - now that
rxrpc_send_data() uses iov_iter primitives, it'll handle ITER_KVEC just
fine.

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