LU-12266 mdd: fix up non-dir creation in SGID dirs

sgid directories have special semantics, making newly created files in
the directory belong to the group of the directory, and newly created
subdirectories will also become sgid. This is historically used for
group-shared directories.

But group directories writable by non-group members should not imply
that such non-group members can magically join the group, so make sure
to clear the sgid bit on non-directories for non-members (but remember
that sgid without group execute means "mandatory locking", just to
confuse things even more).

Adapt fix from inode_init_owner() to use in mdd_create_sanity_check().

Linux-commit: 0fa3ecd87848c9c93c2c828ef4c3a8ca36ce46c7

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Iae253c5cc7865fc81574760ce0ed4d93698b7314
Reviewed-on: https://review.whamcloud.com/34809
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Jenkins
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>