net: udp: remove redundant initialization in udp_gro_complete

The initialization for 'err' with '-ENOSYS' is redundant and
can be removed, as it is updated soon and not used.

Changes since v1:
- Move the err declaration below struct sock *sk

Signed-off-by: Menglong Dong <dong.menglong@zte.com.cn>
Link: https://lore.kernel.org/r/5faa01d5.1c69fb81.8451c.cb5b@mx.google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>