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

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git-svn-id: svn+ssh://svn.freebsd.org/base/head@274274 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
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


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274276 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f