zio: align use of "no dump" flag between use_uma and !use_uma cases

At the moment no ZFS buffers are included into a crash dump unless
ZFS_DEBUG (or INVARIANTS) kernel option is enabled. That's not very
helpful for debugging of ZFS problems, because important information
often resides in metadata buffers.
This change switches the dumping behavior when UMA is used from the
illumos behavior to a more useful behavior that we have on FreeBSD
when ZFS buffers are allocated via malloc.

Reviewed by: smh, mav
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D5892