libspl: ASSERT*: !! for sizeof

sizeof(bitfield.member) is invalid, and this shows up in some FreeBSD
build configurations: work around this by !!ing ‒
this makes the sizeof target the ! result type (_Bool), instead

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Fixes: 42aaf0e ("libspl: ASSERT*: mark arguments as used")
Closes #12984
Closes #12986