EX-5047 pcc: tunable GID to pin/unpin files

EX-5047 pcc: tunable GID to pin/unpin files

In this patch, we adds a tunable "mdt.*.enable_pin_gid" that
controls the GID to pin/unpin files.
Default should be gid=0, so it is limited to root, gid=-1 so any
user can pin/unpin, or gid=<gid> to allow a specific GID (e.g.
wheel).
We add a virtual "lustre.pin" xattr to set/get the xattr for
non-root users, if mdt.*.enable_pin_gid is set correctly.
Add test case sanity-pcc test_104b to verify it.

Change-Id: I1be92f365cfe8dea278559b2c34fcee8e42d1c4d
Signed-off-by: Qian Yingjin <qian@ddn.com>
Reviewed-on: https://review.whamcloud.com/47103
Reviewed-by: Feng, Lei <flei@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>