nvmet: Use PTR_ERR_OR_ZERO() in nvmet_init_discovery()

Simplify this function implementation by using a known function.

Generated by: scripts/coccinelle/api/ptr_ret.cocci
Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>