net/9p: Fix a potential socket leak in p9_socket_open

In the Linux kernel, the following vulnerability has been resolved:

net/9p: Fix a potential socket leak in p9_socket_open

Both p9_fd_create_tcp() and p9_fd_create_unix() will call

p9_socket_open(). If the creation of p9_trans_fd fails,

p9_fd_create_tcp() and p9_fd_create_unix() will return an

error directly instead of releasing the cscoket, which will

result in a socket leak.

This patch adds sock_release() to fix the leak issue.