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>

include/linux/fs.h

@@ -595,6 +595,7 @@ is_uncached_acl(struct posix_acl *acl)
 #define IOP_LOOKUP	0x0002
 #define IOP_NOFOLLOW	0x0004
 #define IOP_XATTR	0x0008
+#define IOP_DEFAULT_READLINK	0x0010
 
 /*
  * Keep mostly read-only and often accessed (especially for