efivarfs: fix error propagation in efivar_entry_get()

efivar_entry_get() always returns success even if the underlying
__efivar_entry_get() fails, masking errors.

This may result in uninitialized heap memory being copied to userspace
in the efivarfs_file_read() path.

Fix it by returning the error from __efivar_entry_get().

Fixes: 2d82e6227ea1 ("efi: vars: Move efivar caching layer into efivarfs")
Cc: <[e-mail]> # v6.1+
Signed-off-by: Kohei Enju <[e-mail]>
Signed-off-by: Ard Biesheuvel <[e-mail]>

fs/efivarfs/vars.c

···
 	err = __efivar_entry_get(entry, attributes, size, data);
 	efivar_unlock();
 
-	return 0;
+	return err;
 }
 
 /**