Cygwin: path: Add fallback for DFS mounted drive.

- If UNC path for DFS is mounted to a drive with drive letter, the
  error "Too many levels of symbolic links" occurs when accessing
  to that drive. This is because GetDosDeviceW() returns unexpected
  string such as "\Device\Mup\DfsClient\;Z:000000000003fb89\dfsserver
  \dfs\linkname" for the mounted UNC path "\??\UNC\fileserver\share".
  This patch adds a workaround for this issue.

  Addresses: https://cygwin.com/pipermail/cygwin/2022-March/250979.html

winsup/cygwin/path.cc

@@ -3527,7 +3527,7 @@ restart:
 		      int remlen = QueryDosDeviceW (drive, remote, MAX_PATH);
 		      if (remlen < 3)
 			goto file_not_symlink; /* fallback */
-		      remlen -= 2;
+		      remlen -= 2; /* Two L'\0' */
 
 		      if (remote[remlen - 1] == L'\\')
 			remlen--;
@@ -3535,20 +3535,27 @@ restart:
 		      UNICODE_STRING rpath;
 		      RtlInitCountedUnicodeString (&rpath, remote,
 						   remlen * sizeof (WCHAR));
+		      const USHORT uncp_len =
+			ro_u_uncp.Length / sizeof (WCHAR) - 1;
 		      if (RtlEqualUnicodePathPrefix (&rpath, &ro_u_uncp, TRUE))
-			remlen -= 6;
+			{
+			  remlen -= uncp_len;
+			  p = remote + uncp_len;
+			}
 		      else if ((p = wcschr (remote, L';'))
 			       && p + 3 < remote + remlen
 			       && wcsncmp (p + 1, drive, 2) == 0
 			       && (p = wcschr (p + 3, L'\\')))
-			remlen -= p - remote - 1;
+			remlen -= p - remote;
 		      else
 			goto file_not_symlink; /* fallback */
+		      if (wcsncasecmp (fpath.Buffer + uncp_len, p, remlen))
+			goto file_not_symlink; /* fallback (not expected) */
 		      /* Hackfest */
 		      fpath.Buffer[4] = drive[0]; /* Drive letter */
 		      fpath.Buffer[5] = L':';
-		      WCHAR *to = fpath.Buffer + 6;
-		      WCHAR *from = to + remlen;
+		      WCHAR *to = fpath.Buffer + 6; /* Next to L':' */
+		      WCHAR *from = fpath.Buffer + uncp_len + remlen;
 		      memmove (to, from,
 			       (wcslen (from) + 1) * sizeof (WCHAR));
 		      fpath.Length -= (from - to) * sizeof (WCHAR);