MFV: r336486

Prevent reinstallation of an already in-use group key.
Upline git commit cb5132bb35698cc0c743e34fe0e845dfc4c3e410.

Obtained from: https://w1.fi/security/2017-1/\

		rebased-v2.6-0002-Prevent-reinstallation-\
		of-an-already-in-use-group-ke.patch

X-MFC-with: r336203