Switch ztest mmap(2) ASSERTs to VERIFYs

This is just a small bit of cleanup to ensure ztest fails early
on systems where mmap(2) is not functioning. For the automated
testing which is the primary consumer of ztest there is no
functional change because debugging is always enabled.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2177