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/file.c b/fs/nfs/file.c
index 41963ff..e679d24 100644
--- a/fs/nfs/file.c
+++ b/fs/nfs/file.c
@@ -623,6 +623,9 @@
 	/* make sure the cache has finished storing the page */
 	nfs_fscache_wait_on_page_write(NFS_I(inode), page);
 
+	wait_on_bit_action(&NFS_I(inode)->flags, NFS_INO_INVALIDATING,
+			nfs_wait_bit_killable, TASK_KILLABLE);
+
 	lock_page(page);
 	mapping = page_file_mapping(page);
 	if (mapping != inode->i_mapping)