crimson: keep seastar's sanitizers in lockstep with WITH_ASAN

crimson: keep seastar's sanitizers in lockstep with WITH_ASAN

Seastar_SANITIZE defaults to "DEFAULT", which turns on ASan and UBSan for
Debug and Sanitize builds regardless of ceph's WITH_ASAN. A Debug build
with WITH_ASAN=OFF then instruments seastar while ceph links tcmalloc, and
the seastar tests crash before main() because ASan calls the allocator via
dlsym before it is initialized.

Force Seastar_SANITIZE to follow WITH_ASAN, next to the other forced
Seastar_* cache variables, so seastar is sanitized only when ceph is. This
keeps the fix on the ceph side instead of carrying a seastar fork patch.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>