The EPOCH array size is 4096 but the driver does not enforce any limit
on ring size, so setting ring size >4096 corrupts adjacent memory
and leads to undefined behavior.

Toggle epoch_bit directly at every ring-producer wrap point (TX/RX
encap and refill, MPC crypto commands, kTLS presync/replay) instead of
looking it up per-index in the doorbell path, dropping the now-unused
epoch_arr[] snapshot array.

Reported by: Drew Gallatin