net: ipa: only reset hashed tables when supported

In the Linux kernel, the following vulnerability has been resolved:

net: ipa: only reset hashed tables when supported

Last year, the code that manages GSI channel transactions switched
from using spinlock-protected linked lists to using indexes into the
ring buffer used for a channel. Recently, Google reported seeing
transaction reference count underflows occasionally during shutdown.

Doug Anderson found a way to reproduce the issue reliably, and
bisected the issue to the commit that eliminated the linked lists
and the lock. The root cause was ultimately determined to be
related to unused transactions being committed as part of the modem
shutdown cleanup activity. Unused transactions are not normally
expected (except in error cases).

The modem uses some ranges of IPA-resident memory, and whenever it
shuts down we zero those ranges. In ipa_filter_reset_table() a
transaction is allocated to zero modem filter table entries. If
hashing is not supported, hashed table memory should not be zeroed.
But currently nothing prevents that, and the result is an unused
transaction. Something similar occurs when we zero routing table
entries for the modem.

By preventing any attempt to clear hashed tables when hashing is not
supported, the reference count underflow is avoided in this case.

Note that there likely remains an issue with properly freeing unused
transactions (if they occur due to errors). This patch addresses
only the underflows that Google originally reported.