osmo_io: Fix misaligment of iofd->cmsg used as struct cmsghdr
Fixes following ASAN runtime error:
"""
src/stream.c:398:47: runtime error: member access within misaligned address 0x516000012b81 for type 'struct cmsghdr', which requires 8 byte alignment
0x516000012b81: note: pointer points here
51 00 00 00 30 00 00 00 00 00 00 00 84 00 00 00 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 03
^
"""
Related: OS#6905
Change-Id: Ia80dfc4dbffe85514b18dcf8d36b85bfafd76d64
