bpf: tcp: Make mem flags configurable through bpf_iter_tcp_realloc_batch

Prepare for the next patch which needs to be able to choose either
GFP_USER or GFP_NOWAIT for calls to bpf_iter_tcp_realloc_batch.

Signed-off-by: Jordan Rife <jordan@jrife.io>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>