9p: adjust 9p vfs write operation

Currently, the 9p net wire operation ensures that all data is sent by sending
multiple packets if the data requested is larger than the msize.  This is
better handled in the vfs code so that we can simplify wire operations to
being concerned with only putting data onto and taking data off of the wire.

Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>