Linux 3.11 compat: fops->iterate()

Commit torvalds/linux@2233f31aad
replaced ->readdir() with ->iterate() in struct file_operations.
All filesystems must now use the new ->iterate method.

To handle this the code was reworked to use the new ->iterate
interface.  Care was taken to keep the majority of changes
confined to the ZPL layer which is already Linux specific.
However, minor changes were required to the common zfs_readdir()
function.

Compatibility with older kernels was accomplished by adding
versions of the trivial dir_emit* helper functions.  Also the
various *_readdir() functions were reworked in to wrappers
which create a dir_context structure to pass to the new
*_iterate() functions.

Unfortunately, the new dir_emit* functions prevent us from
passing a private pointer to the filldir function.  The xattr
directory code leveraged this ability through zfs_readdir()
to generate the list of xattr names.  Since we can no longer
use zfs_readdir() a simplified zpl_xattr_readdir() function
was added to perform the same task.

Signed-off-by: Richard Yao <ryao@cs.stonybrook.edu>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1653
Issue #1591

include/sys/zfs_vnops.h

@@ -31,6 +31,7 @@
 #include <sys/cred.h>
 #include <sys/fcntl.h>
 #include <sys/pathname.h>
+#include <sys/zpl.h>
 
 #ifdef	__cplusplus
 extern "C" {
@@ -51,8 +52,7 @@ extern int zfs_mkdir(struct inode *dip, char *dirname, vattr_t *vap,
     struct inode **ipp, cred_t *cr, int flags, vsecattr_t *vsecp);
 extern int zfs_rmdir(struct inode *dip, char *name, struct inode *cwd,
     cred_t *cr, int flags);
-extern int zfs_readdir(struct inode *ip, void *dirent, filldir_t filldir,
-    loff_t *pos, cred_t *cr);
+extern int zfs_readdir(struct inode *ip, struct dir_context *ctx, cred_t *cr);
 extern int zfs_fsync(struct inode *ip, int syncflag, cred_t *cr);
 extern int zfs_getattr(struct inode *ip, vattr_t *vap, int flag, cred_t *cr);
 extern int zfs_getattr_fast(struct inode *ip, struct kstat *sp);