net: 9p: Correct function names in the kerneldoc comments

Fix the following W=1 kernel build warning(s):

 net/9p/client.c:133: warning: expecting prototype for parse_options(). Prototype was for parse_opts() instead
 net/9p/client.c:269: warning: expecting prototype for p9_req_alloc(). Prototype was for p9_tag_alloc() instead

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>