modpost: Stop grab_file() from leaking filedescriptors if fstat() fails

In case the open() call succeeds but the subsequent fstat() call
fails, then we'll return without close()'ing the filedescriptor.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>