Linux 3.11: Convert from readdir to iterate file operation

Convert the readdir function so that it can be used as the new
"iterate" file operation.  This new operation is passed a context
that contains a pointer to the filldir function and the offset.
The context is passed into the new dir_emit function that will
call the function specified by the context.

The new dir_emit function returns true on success, so we must be
careful about how we check for failure since this is different
behaviour from what filldir currently does.

Change-Id: I6b01b4c78a501bdf4f8d583b0d3b94d677c5d541
Reviewed-on: http://gerrit.openafs.org/10051
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>