ecryptfs: read on a directory should return EISDIR if not supported

read() calls against a file descriptor connected to a directory are
incorrectly returning EINVAL rather than EISDIR:

  [EISDIR]
    [XSI] [Option Start] The fildes argument refers to a directory and the
    implementation does not allow the directory to be read using read()
    or pread(). The readdir() function should be used instead. [Option End]

This occurs because we do not have a .read operation defined for
ecryptfs directories.  Connect this up to generic_read_dir().

BugLink: http://bugs.launchpad.net/bugs/719691


Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Tyler Hicks <tyhicks@linux.vnet.ibm.com>