vfs: default to generic_readlink()

If i_op->readlink is NULL, but i_op->get_link is set then vfs_readlink()
defaults to calling generic_readlink().

The IOP_DEFAULT_READLINK flag indicates that the above conditions are met
and the default action can be taken.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

Documentation/filesystems/porting

@@ -596,3 +596,7 @@ in your dentry operations instead.
 [mandatory]
 	->rename() has an added flags argument.  Any flags not handled by the
         filesystem should result in EINVAL being returned.
+--
+[recommended]
+	->readlink is optional for symlinks.  Don't set, unless filesystem needs
+	to fake something for readlink(2).

Documentation/filesystems/vfs.txt

@@ -451,9 +451,6 @@ otherwise noted.
 	exist; this is checked by the VFS.  Unlike plain rename,
 	source and target may be of different type.
 
-  readlink: called by the readlink(2) system call. Only required if
-	you want to support reading symbolic links
-
   get_link: called by the VFS to follow a symbolic link to the
 	inode it points to.  Only required if you want to support
 	symbolic links.  This method returns the symlink body
@@ -468,6 +465,12 @@ otherwise noted.
 	argument.  If request can't be handled without leaving RCU mode,
 	have it return ERR_PTR(-ECHILD).
 
+  readlink: this is now just an override for use by readlink(2) for the
+	cases when ->get_link uses nd_jump_link() or object is not in
+	fact a symlink.  Normally filesystems should only implement
+	->get_link for symlinks and readlink(2) will automatically use
+	that.
+
   permission: called by the VFS to check for access rights on a POSIX-like
   	filesystem.