net: 9p: initialize sun_server.sun_path to have addr's value only when addr is valid

In p9_fd_create_unix, checking is performed to see if the addr (passed
as an argument) is NULL or not.
However, no check is performed to see if addr is a valid address, i.e.,
it doesn't entirely consist of only 0's.
The initialization of sun_server.sun_path to be equal to this faulty
addr value leads to an uninitialized variable, as detected by KMSAN.
Checking for this (faulty addr) and returning a negative error number
appropriately, resolves this issue.

Link: http://lkml.kernel.org/r/20201012042404.2508-1-anant.thazhemadam@gmail.com


Reported-by:  <syzbot+75d51fe5bf4ebe988518@syzkaller.appspotmail.com>
Tested-by:  <syzbot+75d51fe5bf4ebe988518@syzkaller.appspotmail.com>
Signed-off-by: Anant Thazhemadam <anant.thazhemadam@gmail.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>