highmem: Provide generic variant of kmap_atomic*

The kmap_atomic* interfaces in all architectures are pretty much the same
except for post map operations (flush) and pre- and post unmap operations.

Provide a generic variant for that.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linuxfoundation.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: https://lore.kernel.org/r/20201103095857.175939340@linutronix.de