ovl: update S_ISGID when setting posix ACLs

ovl: update S_ISGID when setting posix ACLs

This change fixes xfstest generic/375, which failed to clear the
setgid bit in the following test case on overlayfs:

  touch $testfile
  chown 100:100 $testfile
  chmod 2755 $testfile
  _runas -u 100 -g 101 -- setfacl -m u::rwx,g::rwx,o::rwx $testfile

Reported-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Tested-by: Amir Goldstein <amir73il@gmail.com>
Fixes: 87c4b178742b ("ovl: fix POSIX ACL setting")
Cc: <stable@vger.kernel.org> # v4.8