erasure-code/jerasure: Fix thread safety in plugin init

erasure-code/jerasure: Fix thread safety in plugin init

The Jerasure function cauchy_good_general_coding_matrix()
has a race condition where it uses a global variable cbest_init without
proper synchronization. When multiple threads initialize PGs simultaneously,
they can race on this initialization, causing one thread to read from
cbest_all before it's fully initialized, resulting in a segmentation fault.

This commit adds a static mutex to serialize the prepare() calls during init
for all Jerasure techniques.

Fixes: https://tracker.ceph.com/issues/73246
Signed-off-by: Jamie Pryde <jamiepry@uk.ibm.com>