Sebastian discovered that SLQB can enable interrupts early in boot due
to taking the rwsem. It should not be contended here, so XADD algorithm
implementations should not be affected, however spinlock algorithm
implementations do a spin_lock_irq in down_write fastpath and would be
affected.

Move the lock out of early init path, comment why. This also covers
a very small (and basically insignificant) race where kmem_cache_create_ok
checks succeed but kmem_cache_create still creates a duplicate named
cache because the lock was dropped and retaken.
Reported-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Tested-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>