There might be cases where multiple requests trigger the key generation
at the same time and the instance ends up with a non-fitting
public/private key pair. Therefore the whole key generation should be
locked. Other than that this makes sure that user key generation return
values are properly validated.

Signed-off-by: Julius Härtl <jus@bitgrid.net>

Signed-off-by: Julius Härtl <jus@bitgrid.net>

Signed-off-by: Morris Jobke <hey@morrisjobke.de>