Remove .readdir from zpl_file_operations table

The zpl_readdir() function shouldn't be registered as part of
the zpl_file_operations table, it must only be part of the
zpl_dir_file_operations table. By removing this callback
the VFS will now correctly return ENOTDIR when calling
getdents() on a file.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1404