MFV r353561: 10343 ZoL: Prefix all refcount functions with zfs_

MFV r353561: 10343 ZoL: Prefix all refcount functions with zfs_

illumos/illumos-gate@e914ace2e9d9bf2dbf9a1f1ce81cb776022096f5
https://github.com/illumos/illumos-gate/commit/e914ace2e9d9bf2dbf9a1f1ce81cb776022096f5

https://www.illumos.org/issues/10343
  On the openzfs feature/porting matrix, this is listed as:
  prefix to refcount funcs/types
  Having these changes will make it easier to share other work across the
  different ZFS operating systems.
  PR 7963 424fd7c3e Prefix all refcount functions with zfs_
  PR 7885 & 7932 c13060e47 Linux 4.19-rc3+ compat: Remove refcount_t compat
  PR 5823 & 5842 4859fe796 Linux 4.11 compat: avoid refcount_t name conflict

Author: Tim Schumacher <timschumi@gmx.de>
Obtained from:	illumos, ZoL
MFC after:	3 weeks