MFC r341297
sfxge(4): fix out of bounds read when dereferencing sdup
Introduce and use macro to make sure that MCDI buffers allocated
on stack are rounded up properly.
Submitted by: Gautam Dawar <gdawar at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18259