net: ipa: zero unused portions of filter table memory

Not all filter table entries are used.  Only certain endpoints
support filtering, and the table begins with a bitmap indicating
which endpoints use the "slots" that follow for filter rules.

Currently, unused filter table entries are not initialized.
Instead, zero-fill the entire unused portion of the filter table
memory regions, to make it more obvious that memory is unused (and
not subsequently modified).

This is not strictly necessary, but the result is reassuring when
looking at filter table memory.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>