MFV r272802:

 - Limit ARC for zdb at 256MB.  zdb do not typically revisit data
   in the ARC.
 - Increase default max_inflight from 200 to 1000 (can be overriden
   by -I) so we can queue more I/Os when doing scrubbing.
 - Print status while loading meataslabs for leak detection.

Illumos issues:

    5169 zdb should limit its ARC size
    5170 zdb -c should create more scrub i/os by default
    5171 zdb should print status while loading metaslabs for leak detection

MFC after:	2 weeks


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272806 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f