glibc 2.25 compat: remove assert(X=Y)

glibc 2.25 compat: remove assert(X=Y)

The assert() related definitions in glibc 2.25 were altered to warn
about assert(X=Y) when -Wparentheses is used.  See
https://abi-laboratory.pro/tracker/changelog/glibc/2.25/log.html

lib/list.c used this construct to set the value of a magic field which
is defined only when debugging.

Replaced the assert()s with #ifndef/#endifs.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Closes #610