isofs: implement dmode option

Implement dmode option for iso9660 filesystem to allow setting of access
rights for directories on the filesystem.

Signed-off-by: Jan Kara <jack@suse.cz>
Cc: "Ilya N. Golubev" <gin@mo.msk.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/filesystems/isofs.txt

@@ -24,6 +24,7 @@ Mount options unique to the isofs filesystem.
   map=normal    Map non-Rock Ridge filenames to lower case
   map=acorn     As map=normal but also apply Acorn extensions if present
   mode=xxx      Sets the permissions on files to xxx
+  dmode=xxx     Sets the permissions on directories to xxx
   nojoliet      Ignore Joliet extensions if they are present.
   norock        Ignore Rock Ridge extensions if they are present.
   hide		Completely strip hidden files from the file system.