pass a struct path to vfs_statfs

We'll need the path to implement the flags field for statvfs support.
We do have it available in all callers except:

 - ecryptfs_statfs.  This one doesn't actually need vfs_statfs but just
   needs to do a caller to the lower filesystem statfs method.
 - sys_ustat.  Add a non-exported statfs_by_dentry helper for it which
   doesn't won't be able to fill out the flags field later on.

In addition rename the helpers for statfs vs fstatfs to do_*statfs instead
of the misleading vfs prefix.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/cachefiles/daemon.c b/fs/cachefiles/daemon.c
index c241356..24eb0d3 100644
--- a/fs/cachefiles/daemon.c
+++ b/fs/cachefiles/daemon.c
@@ -683,6 +683,10 @@
 			 unsigned fnr, unsigned bnr)
 {
 	struct kstatfs stats;
+	struct path path = {
+		.mnt	= cache->mnt,
+		.dentry	= cache->mnt->mnt_root,
+	};
 	int ret;
 
 	//_enter("{%llu,%llu,%llu,%llu,%llu,%llu},%u,%u",
@@ -697,7 +701,7 @@
 	/* find out how many pages of blockdev are available */
 	memset(&stats, 0, sizeof(stats));
 
-	ret = vfs_statfs(cache->mnt->mnt_root, &stats);
+	ret = vfs_statfs(&path, &stats);
 	if (ret < 0) {
 		if (ret == -EIO)
 			cachefiles_io_error(cache, "statfs failed");