Always allocate the atid table during attach.

[jhb]

Summary

Previously the table was allocated on first use by TOE and the ratelimit
code. The forthcoming NIC KTLS code also uses this table. Allocate it
unconditionally during attach to simplify consumers.

Test Plan

• the NIC KTLS branch used this for a long time
• I tried to test in isolation on head, but head panics due to epoch changes