NFSv4.1: Clean ups and bugfixes for the pNFS read/writeback/commit code

Move more pnfs-isms out of the generic commit code.

Bugfixes:

- filelayout_scan_commit_lists doesn't need to get/put the lseg.
  In fact since it is run under the inode->i_lock, the lseg_put()
  can deadlock.

- Ensure that we distinguish between what needs to be done for
  commit-to-data server and what needs to be done for commit-to-MDS
  using the new flag PG_COMMIT_TO_DS. Otherwise we may end up calling
  put_lseg() on a bucket for a struct nfs_page that got written
  through the MDS.

- Fix a case where we were using list_del() on an nfs_page->wb_list
  instead of list_del_init().

- filelayout_initiate_commit needs to call filelayout_commit_release
  on error instead of the mds_ops->rpc_release(). Otherwise it won't
  clear the commit lock.

Cleanups:

- Let the files layout manage the commit lists for the pNFS case.
  Don't expose stuff like pnfs_choose_commit_list, and the fact
  that the commit buckets hold references to the layout segment
  in common code.

- Cast out the put_lseg() calls for the struct nfs_read/write_data->lseg
  into the pNFS layer from whence they came.

- Let the pNFS layer manage the NFS_INO_PNFS_COMMIT bit.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: Fred Isaman <iisaman@netapp.com>

include/linux/nfs_page.h

@@ -28,6 +28,7 @@ enum {
 	PG_NEED_COMMIT,
 	PG_NEED_RESCHED,
 	PG_PARTIAL_READ_FAILED,
+	PG_COMMIT_TO_DS,
 };
 
 struct nfs_inode;
@@ -104,6 +105,16 @@ nfs_lock_request_dontget(struct nfs_page *req)
 	return !test_and_set_bit(PG_BUSY, &req->wb_flags);
 }
 
+static inline int
+nfs_lock_request(struct nfs_page *req)
+{
+	if (test_and_set_bit(PG_BUSY, &req->wb_flags))
+		return 0;
+	kref_get(&req->wb_kref);
+	return 1;
+}
+
+
 /**
  * nfs_list_add_request - Insert a request into a list
  * @req: request