crypto: testmgr - Only disable migration in crypto_disable_simd_for_test()

crypto_disable_simd_for_test() disables preemption in order to receive a
stable per-CPU variable which it needs to modify in order to alter
crypto_simd_usable() results.

This can also be achived by migrate_disable() which forbidds CPU
migrations but allows the task to be preempted. The latter is important
for PREEMPT_RT since operation like skcipher_walk_first() may allocate
memory which must not happen with disabled preemption on PREEMPT_RT.

Use migrate_disable() in crypto_disable_simd_for_test() to achieve a
stable per-CPU pointer.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>