Changes

Correctly calculate size iofd_msgb_alloc2()

The storage size of 'headroom' must be larger than 16 bits. Otherwise a
value above 65535 cannot be stored correctly and cause wrong
calculation of msgb size.

Related: Coverity CID#548925
Change-Id: I16c1e08fb64a1fafaeee1844fd8c00ecc2861d12