NFS: Don't write enable new pages while an invalidation is proceeding

nfs_vm_page_mkwrite() should wait until the page cache invalidation
is finished. This is the second patch in a 2 patch series to deprecate
the NFS client's reliance on nfs_release_page() in the context of
nfs_invalidate_mapping().

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
diff --git a/fs/nfs/inode.c b/fs/nfs/inode.c
index 8edb7d0..d42dff6 100644
--- a/fs/nfs/inode.c
+++ b/fs/nfs/inode.c
@@ -1035,6 +1035,7 @@
 
 	if (mapping->nrpages != 0) {
 		if (S_ISREG(inode->i_mode)) {
+			unmap_mapping_range(mapping, 0, 0, 0);
 			ret = nfs_sync_mapping(mapping);
 			if (ret < 0)
 				return ret;