devlink: fix memory leak in cmd_dev_flash()

devlink: fix memory leak in cmd_dev_flash()

nlg_ntf is dinamically allocated in mnlg_socket_open(), and is freed on
the out: return path. However, some error paths do not free it,
resulting in memory leak.

This commit fix this using mnlg_socket_close(), and reporting the
correct error number when required.

Fixes: 9b13cddfe268 ("devlink: implement flash status monitoring")
Signed-off-by: Andrea Claudi <aclaudi@redhat.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>