Illumos 5045 - use atomic_{inc,dec}_* instead of atomic_add_*

Illumos 5045 - use atomic_{inc,dec}_* instead of atomic_add_*

5045 use atomic_{inc,dec}_* instead of atomic_add_*
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Garrett D'Amore <garrett@damore.org>
Approved by: Robert Mustacchi <rm@joyent.com>

References:
  https://www.illumos.org/issues/5045
  https://github.com/illumos/illumos-gate/commit/1a5e258

Porting notes:
- All changes to non-ZFS files dropped.
- Changes to zfs_vfsops.c dropped because they were Illumos specific.

Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4220