Libmfx requires 16 bytes aligned input/output for uploading.
Currently only output is 16 byte aligned and assigning same width/height to
input with smaller buffer size actually, thus definitely will cause segment fault.

Can reproduce with any 1080p nv12 rawvideo input:
ffmpeg -init_hw_device qsv=qsv:hw -hwaccel qsv -filter_hw_device qsv -f rawvideo -pix_fmt nv12 -s:v 1920x1080
-i 1080p_nv12.yuv -vf 'format=nv12,hwupload=extra_hw_frames=16,hwdownload,format=nv12' -an -y out_nv12.yuv

It can fix #7418

Signed-off-by: Zhong Li <zhong.li@intel.com>