[linux] fix: memory-mapped file, stat: No such file or directory

In old linux kernel (e.g. linux-2.6) which does not have this feature: Commit
6b4e306aa3dc ("ns: proc files for namespace naming policy."), means this path
"/proc/self/ns" is not existed. Since lsof-4.96.0 with Commit dbad150 ("
[linux] obtain correct information of memory-mapped file."), compare_mntns()
would misunderstand it is in a different mount namespace if "/proc/self/ns" is
not existed, returns -1, go through map_files lookup, and finally lead to lsof
gets failed with error message "stat: No such file or directory".

If "/proc/self/ns" or its underlying path is not existed, compare_mntns()
returns 0 instead of -1, in order to go through stat_directly as old days.

Signed-off-by: Jones Syue <[email protected]>

lib/dialects/linux/dproc.c

@@ -1370,14 +1370,14 @@ static int compare_mntns(int pid) /* pid of the target process */
     int ret;
 
     if (stat("/proc/self/ns/mnt", &sb_self))
-        return -1;
+        return 0;
 
     ret = snprintf(nspath, sizeof(nspath), "/proc/%d/ns/mnt", pid);
     if (ret >= sizeof(nspath) || ret <= 0)
-        return -1;
+        return 0;
 
     if (stat(nspath, &sb_target))
-        return -1;
+        return 0;
 
     if (sb_self.st_ino != sb_target.st_ino)
         return -1;