mm/slab: do not access current->mems_allowed_seq if !allow_spin

mm/slab: do not access current->mems_allowed_seq if !allow_spin

Lockdep complains when get_from_any_partial() is called in an NMI
context, because current->mems_allowed_seq is seqcount_spinlock_t and
not NMI-safe:

  ================================
  WARNING: inconsistent lock state
  6.19.0-rc5-kfree-rcu+ #315 Tainted: G                 N
  --------------------------------
  inconsistent {INITIAL USE} -> {IN-NMI} usage.
  kunit_try_catch/9989 [HC1[1]:SC0[0]:HE0:SE1] takes:
  ffff889085799820 (&____s->seqcount#3){.-.-}-{0:0}, at: ___slab_alloc+0x58f/0xc00
  {INITIAL USE} state was registered at:
    lock_acquire+0x185/0x320
    kernel_init_freeable+0x391/0x1150
    kernel_init+0x1f/0x220
    ret_from_fork+0x736/0x8f0
    ret_from_fork_asm+0x1a/0x30
  irq event stamp: 56
  hardirqs last  enabled at (55): [<ffffffff850a68d7>] _raw_spin_unlock_irq+0x27/0x70
  hardirqs last disabled at (56): [<ffffffff850858ca>] __schedule+0x2a8a/0x6630
  softirqs last  enabled at (0): [<ffffffff81536711>] copy_process+0x1dc1/0x6a10
  softirqs last disabled at (0): [<0000000000000000>] 0x0

  other info that might help us debug this:
   Possible unsafe locking scenario:

         CPU0
         ----
    lock(&____s->seqcount#3);
    <Interrupt>
      lock(&____s->seqcount#3);

   *** DEADLOCK ***

According to Documentation/locking/seqlock.rst, seqcount_t is not
NMI-safe and seqcount_latch_t should be used when read path can interrupt
the write-side critical section. In this case, do not access
current->mems_allowed_seq and avoid retry.

Fixes: af92793e52c3 ("slab: Introduce kmalloc_nolock() and kfree_nolock().")
Cc: stable@vger.kernel.org
Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
Link: https://patch.msgid.link/20260210081900.329447-2-harry.yoo@oracle.com
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>