cifs: have find_readable/writable_file filter by fsuid

When we implement multiuser mounts, we'll need to filter filehandles
by fsuid. Add a flag for multiuser mounts and code to filter by
fsuid when it's set.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>
diff --git a/fs/cifs/dir.c b/fs/cifs/dir.c
index 5adf47f..e249b56 100644
--- a/fs/cifs/dir.c
+++ b/fs/cifs/dir.c
@@ -144,6 +144,7 @@
 
 	pCifsFile->netfid = fileHandle;
 	pCifsFile->pid = current->tgid;
+	pCifsFile->uid = current_fsuid();
 	pCifsFile->pInode = igrab(newinode);
 	pCifsFile->mnt = mnt;
 	pCifsFile->pfile = file;