ike-sa-manager: Safely access the RNG instance with an rwlock

ike-sa-manager: Safely access the RNG instance with an rwlock

Threads might still be allocating SPIs (e.g. triggered by an acquire or
an inbound message) while the main thread calls flush().  If there is a
context switch right after such a thread successfully checked this->rng
in get_spi() and the main thread destroys the RNG instance right then,
that worker thread will cause a segmentation fault when it continues and
attempts to call get_bytes().

Fixes #1014.