crypto: jitterentropy - replace long-held spinlock with mutex

jent_kcapi_random() serializes the shared jitterentropy state, but it
currently holds a spinlock across the jent_read_entropy() call. That
path performs expensive jitter collection and SHA3 conditioning, so
parallel readers can trigger stalls as contending waiters spin for
the same lock.

To prevent non-preemptible lock hold, replace rng->jent_lock with a
mutex so contended readers sleep instead of spinning on a shared lock
held across expensive entropy generation.

Fixes: bb5530e40824 ("crypto: jitterentropy - add jitterentropy RNG")
Reported-by: Yifan Wu <yifanwucs@gmail.com>
Reported-by: Juefei Pu <tomapufckgml@gmail.com>
Reported-by: Yuan Tan <yuantan098@gmail.com>
Suggested-by: Xin Liu <bird@lzu.edu.cn>
Signed-off-by: Haixin Xu <jerryxucs@gmail.com>
Reviewed-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

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