PTW protocol violation if during pipeline flush

[zarubaf]

The PTW has an illegal state in which it can violate the req, gnt protocol (from PTE_LOOKUP to IDLE during flush).

https://github.com/pulp-platform/ariane/blob/838b04a96c45cee81ce658492baf74f117357dd2/src/ptw.sv#L331-L338

Example execution trace:

1127755000     112766 U 0000000000005fce 0 00000013 nop                                 
1127765000     112767 U 0000000000005fd0 0 00ee579b srliw            a5, t3, 0xe         x15 =0000000000000000 t3  :0000000000000000
1127775000     112768 U 0000000000005fd4 0 0000100f fence.i                             
1132915000     113282 U 0000000000005fd8 0 40755b3b sraw             s6, a0, t2          x22 =ffffffffffffffff a0  :ffffffffffffffff t2  :0000000000000000
1132925000     113283 U 0000000000005fdc 0 12000073 sfence.vma                          
1133245000     113315 U 0000000000005fe0 0 00000000 INVALID                             
Exception @1133245000, PC: 0000000000005fe0, Cause: Instruction Page Fault, 
tval: 0000000000005fe0