illumos gate

From Matt Ahrens's bug report at Delphix:

mdb_module() returns the wrong module when called from the callback of
mdb_pwalk() -- it returns the module that the walker is in, rather than the
module the callback is in.  This causes mdb_ctf_vread() to fail because it
looks up the wrong module's CTF info.

Steps to reproduce:

add these lines to a module (e.g. zfs.c):

struct modtest {
    int i;
};

/* ARGSUSED */
static int
modtest_cb(uintptr_t addr, const void *ignored, void *arg)
{
    mdb_ctf_id_t id;
    mdb_printf("from callback: %d\\n",
        mdb_ctf_module_lookup("struct modtest", &id));

    return (WALK_DONE);
}

/* ARGSUSED */
static int
modtest(uintptr_t addr, uint_t flags, int argc, const mdb_arg_t *argv)
{
    mdb_ctf_id_t id;
    mdb_printf("from dcmd: %d\\n",
        mdb_ctf_module_lookup("struct modtest", &id));

    mdb_pwalk("list", modtest_cb, NULL, addr);
    return (DCMD_OK);
}

register the dcmd:
    { "modtest", ":", "test mdb_module()", modtest},

add this line to mapfile-extern:

        mdb_ctf_module_lookup        { FLAGS = EXTERN };

Run by providing a list_t*, like so:

> ARC_mru::modtest
from dcmd: 0
from callback: -1

Results should be 0 from both dcmd and callback.