highmem: don't disable preemption on RT in kmap_atomic()

highmem: don't disable preemption on RT in kmap_atomic()

kmap_atomic() disables preemption and pagefaults for historical reasons.
The conversion to kmap_local(), 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 kmap_atomic() semantics are problematic
due to the implicit disabling of preemption which makes it impossible to
acquire 'sleeping' spinlocks within the kmap 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.

Link: https://lkml.kernel.org/r/20210810091116.pocdmaatdcogvdso@linutronix.de
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>