padata: downgrade padata_do_multithreaded to serial execution for non-SMP

padata: downgrade padata_do_multithreaded to serial execution for non-SMP

hugetlb parallelization depends on PADATA, and PADATA depends on SMP.

PADATA consists of two distinct functionality: One part is
padata_do_multithreaded which disregards order and simply divides tasks
into several groups for parallel execution.  Hugetlb init parallelization
depends on padata_do_multithreaded.

The other part is composed of a set of APIs that, while handling data in
an out-of-order parallel manner, can eventually return the data with
ordered sequence.  Currently Only `crypto/pcrypt.c` use them.

All users of PADATA of non-SMP case currently only use
padata_do_multithreaded.  It is easy to implement a serial one in
include/linux/padata.h.  And it is not necessary to implement another
functionality unless the only user of crypto/pcrypt.c does not depend on
SMP in the future.

Link: https://lkml.kernel.org/r/20240222140422.393911-6-gang.li@linux.dev
Signed-off-by: Gang Li <ligang.bdlg@bytedance.com>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Acked-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Jane Chu <jane.chu@oracle.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>