- Allow to change vfs.zfs.arc_meta_limit at runtime.
- Change vfs.zfs.arc_meta_used from CTLFLAG_RDTUN to CTLFLAG_RD, as it is
  not a tunable.

MFC after:	3 days

(cherry picked from commit 6cd4b91f813aa3c0e4d3486fddf5dac445965cd2)