ext4: implement exists() for ext4fs

This hooks into the generic "file exists" support added in an earlier
patch, and provides an implementation for the ext4 filesystem.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/fs/fs.c b/fs/fs.c
index 2e60f57..4e9b162 100644
--- a/fs/fs.c
+++ b/fs/fs.c
@@ -102,7 +102,7 @@
 		.probe = ext4fs_probe,
 		.close = ext4fs_close,
 		.ls = ext4fs_ls,
-		.exists = fs_exists_unsupported,
+		.exists = ext4fs_exists,
 		.read = ext4_read_file,
 		.write = fs_write_unsupported,
 	},