9p: use the updated offset given by generic_write_checks

Without this fix, even if a file is opened in O_APPEND mode, data will be
written at current file position instead of end of file.

Signed-off-by: M. Mohan Kumar <mohan@in.ibm.com>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>