LU-8851 nodemap: add uid/gid only flags to control mapping

LU-8851 nodemap: add uid/gid only flags to control mapping

This patch adds two flags to nodemaps which control whether or not
the nodemap should map UIDs, GIDs, or both. These flags can be
controlled via lctl as a new nodemap parameter map_mode, with values
both, uid_only, or gid_only.

Signed-off-by: Kit Westneat <kit.westneat@gmail.com>
Change-Id: I3efe6ff348d909c196a89273a0c9c046c56dbf1d
Reviewed-on: https://review.whamcloud.com/23853
Reviewed-by: Chris Hanna <hannac@iu.edu>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Stephan Thiell <sthiell@stanford.edu>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>