ifindex_alloc_locked() never fails and doesn't have no-lock version,
so change the prototype.

Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.
MFV r274271:

Improve zdb -b performance:

 - Reduce gethrtime() call to 1/100th of blkptr's;
 - Skip manipulating the size-ordered tree;
 - Issue more (10, previously 3) async reads;
 - Use lighter weight testing in traverse_visitbp();

Illumos issue:
    5243 zdb -b could be much faster

MFC after:	2 weeks