mempool: clarify behavior of mempool_alloc_preallocated()

mempool: clarify behavior of mempool_alloc_preallocated()

The documentation of that function promises to never sleep.  However on
PREEMPT_RT a spinlock_t might in fact sleep.

Reword the documentation so users can predict its behavior better.

mempool could also replace spinlock_t with raw_spinlock_t which doesn't
sleep even on PREEMPT_RT but that would take away the improved
preemptibility of sleeping locks.

Link: https://lkml.kernel.org/r/20251014-mempool-doc-v1-1-bc9ebf169700@linutronix.de
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Christoph Lameter <cl@gentwo.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: "Vishal Moola (Oracle)" <vishal.moola@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>