IB/mthca: Use memset_startat() for clearing mpt_entry

In preparation for FORTIFY_SOURCE performing compile-time and run-time
field bounds checking for memset(), avoid intentionally writing across
neighboring fields.

Use memset_startat() so memset() doesn't get confused about writing beyond
the destination member that is intended to be the starting point of
zeroing through the end of the struct.

Link: https://lore.kernel.org/r/20211213223331.135412-15-keescook@chromium.org


Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>