Rename the rand_drbg_st data member "pool" to "seed_pool"

... to make the intended use more clear and differentiate
it from the data member "adin_pool".

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/7575)

(cherry picked from commit 31f32abb8eb2a3ebd8500e6e0460b4a6791e5ed7)