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