ovl: select EXPORTFS

We get a link error when EXPORTFS is not enabled:

ERROR: "exportfs_encode_fh" [fs/overlayfs/overlay.ko] undefined!
ERROR: "exportfs_decode_fh" [fs/overlayfs/overlay.ko] undefined!

This adds a Kconfig 'select' statement for overlayfs, the same way that
it is done for the other users of exportfs.

Fixes: 3a1e819b ("ovl: store file handle of lower inode on copy up")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/Kconfig

 config OVERLAY_FS
 	tristate "Overlay filesystem support"
+	select EXPORTFS
 	help
 	  An overlay filesystem combines two filesystems - an 'upper' filesystem
 	  and a 'lower' filesystem.  When a name exists in both filesystems, the