MFV r287703, r287705 (by delphij):
6091 avl_add doesn't assert on non-debug builds

Use assfail() from libuutil instead of ASSERT() in userland
AVL avl_add.

illumos/illumos-gate@faa2b6be2fc102adf9ed584fc1a667b4ddf50d78

Illumos issues:

6091 avl_add doesn't assert on non-debug builds
https://www.illumos.org/issues/6091