io-mapping: don't disable preempt on RT in io_mapping_map_atomic_wc().

io_mapping_map_atomic_wc() disables preemption and pagefaults for
historical reasons.  The conversion to io_mapping_map_local_wc(), which
only disables migration, cannot be done wholesale because quite some call
sites need to be updated to accommodate with the changed semantics.

On PREEMPT_RT enabled kernels the io_mapping_map_atomic_wc() semantics are
problematic due to the implicit disabling of preemption which makes it
impossible to acquire 'sleeping' spinlocks within the mapped atomic
sections.

PREEMPT_RT replaces the preempt_disable() with a migrate_disable() for
more than a decade.  It could be argued that this is a justification to do
this unconditionally, but PREEMPT_RT covers only a limited number of
architectures and it disables some functionality which limits the coverage
further.

Limit the replacement to PREEMPT_RT for now.  This is also done
kmap_atomic().

Link: https://lkml.kernel.org/r/20230310162905.O57Pj7hh@linutronix.de


Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reported-by: Richard Weinberger <richard.weinberger@gmail.com>
  Link: https://lore.kernel.org/CAFLxGvw0WMxaMqYqJ5WgvVSbKHq2D2xcXTOgMCpgq9nDC-MWTQ@mail.gmail.com


Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>