Add a new tunable/sysctl, vfs.zfs.free_max_blocks, which can be used to
limit how many blocks can be free'ed before a new transaction group is
created. The default is no limit (infinite), but we should probably have
a lower default, e.g. 100,000.
With this limit, we can guard against the case where ZFS could run out of
memory when destroying large numbers of blocks in a single transaction
group, as the entire DDT needs to be brought into memory.
Illumos issue:
5138 add tunable for maximum number of blocks freed in one txg
MFC after: 2 weeks
be1b14a063