OpenZFS 9424 - ztest failure: "unprotected error in call to Lua API (Invalid value...

OpenZFS 9424 - ztest failure: "unprotected error in call to Lua API (Invalid value type 'function' for key 'error')"

Ztest failed with the following crash.

    ::status

    debugging core file of ztest (64-bit) from clone-dc-slave-280-bc7947b1.dcenter
    file: /usr/bin/amd64/ztest
    initial argv: /usr/bin/amd64/ztest
    threading model: raw lwps
    status: process terminated by SIGABRT (Abort), pid=2150 uid=1025 code=-1
    panic message: failure for thread 0xfffffd7fff112a40, thread-id 1: unprotected error in call to Lua API (Invalid
    value type 'function' for key 'error')

    ::stack

    libc.so.1`_lwp_kill+0xa()
    libc.so.1`_assfail+0x182(fffffd7fffdfe8d0, 0, 0)
    libc.so.1`assfail+0x19(fffffd7fffdfe8d0, 0, 0)
    libzpool.so.1`vpanic+0x3d(fffffd7ffaa58c20, fffffd7fffdfeb00)
    0xfffffd7ffaa28146()
    0xfffffd7ffaa0a109()
    libzpool.so.1`luaD_throw+0x86(3011a48, 2)
    0xfffffd7ffa9350d3()
    0xfffffd7ffa93e3f1()
    libzpool.so.1`zcp_lua_to_nvlist+0x33(3011a48, 1, 2686470, fffffd7ffaa2e2c3)
    libzpool.so.1`zcp_convert_return_values+0xa4(3011a48, 2686470, fffffd7ffaa2e2c3, fffffd7fffdfedd0)
    libzpool.so.1`zcp_pool_error+0x59(fffffd7fffdfedd0, 1e0f450)
    libzpool.so.1`zcp_eval+0x6f8(1e0f450, fffffd7ffaa483f8, 1, 0, 6400000, 1d33b30)
    libzpool.so.1`dsl_destroy_snapshots_nvl+0x12c(2786b60, 0, 484750)
    libzpool.so.1`dsl_destroy_snapshot+0x4f(fffffd7fffdfef70, 0)
    ztest_dsl_dataset_cleanup+0xea(fffffd7fffdff4c0, 1)
    ztest_dataset_destroy+0x53(1)
    ztest_run+0x59f(fffffd7fff0e0498)
    main+0x7ff(1, fffffd7fffdffa88)
    _start+0x6c()

The problem is that zcp_convert_return_values() assumes that there's
exactly one value on the stack, but that isn't always true. It ends up
putting the wrong thing on the stack which is then consumed by
zcp_convert_return values, which either adds the wrong message to the
nvlist, or blows up.

The fix is to make sure that callers of zcp_convert_return_values()
clear the stack before pushing their error message, and
zcp_convert_return_values() should VERIFY that the stack is the expected
size.

Authored by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Sebastien Roy <sebastien.roy@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Don Brady <don.brady@delphix.com>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
Approved by: Robert Mustacchi <rm@joyent.com>

OpenZFS-issue: https://www.illumos.org/issues/9424
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/eb7e57429
Closes #7696