staging: lustre: lov: fix dereference of ERR_PTR

If lov_sub_get() fails then it returns the error code in ERR_PTR, but
here we were dereferencing sub without checking if lov_sub_get() has
actually succeeded or not. And on error we can directly return the error
code from lov_io_fault_start() as it return 0 on success and the error
code on error.

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>