serial2002: switch serial2002_tty_write to kernel_{read/write}

Instead of playing games with the address limit.  This also gains
us proper usage of the write counter, time stamp updates and kvec
validation.

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