MFV r247174:
Expose arc_meta_limit, et al via kstats.
Note that as a result, vfs.zfs.arc_meta_used is removed.
The existing vfs.zfs.arc_meta_limit sysctl/tunable is retained
with a SYSCTL_PROC wrapper.
Illumos ZFS issues:
3561 arc_meta_limit should be exposed via kstats
Relnotes: yes
MFC after: 2 weeks