scsi: libfc: Remove get_cpu() semantics in fc_exch_em_alloc()

The get_cpu() in fc_exch_em_alloc() was introduced in commit f018b73af6db
("[SCSI] libfc, libfcoe, fcoe: use smp_processor_id() only when preempt
disabled") for no other reason than to simply use smp_processor_id()
without getting a warning, because everything is done with the pool->lock
held anyway. However, get_cpu(), by disabling preemption, does not play
well with PREEMPT_RT, particularly when acquiring a regular (and thus
sleepable) spinlock.

Therefore remove the get_cpu() and just use the unstable value as we will
have CPU locality guarantees next by taking the lock. The window of
migration, as noted by Sebastian, is small and even if it happens the
result is correct.

Link: https://lore.kernel.org/r/[e-mail]
Link: https://lore.kernel.org/r/[e-mail]
Acked-by: Sebastian Andrzej Siewior <[e-mail]>
Signed-off-by: Davidlohr Bueso <[e-mail]>
Signed-off-by: Sebastian Andrzej Siewior <[e-mail]>
Signed-off-by: Martin K. Petersen <[e-mail]>

drivers/scsi/libfc/fc_exch.c

···
 	}
 	memset(ep, 0, sizeof(*ep));
 
-	cpu = get_cpu();
+	cpu = raw_smp_processor_id();
 	pool = per_cpu_ptr(mp->pool, cpu);
 	spin_lock_bh(&pool->lock);
-	put_cpu();
 
 	/* peek cache of free slot */
 	if (pool->left != FC_XID_UNKNOWN) {