mm: move more expensive part of XA setup out of mapping check

The fast path here is not needing any writeback, yet we spend time
setting up the xarray lookup data upfront.  Move the part that actually
needs to iterate the address space mapping into a separate helper,
saving ~30% of the time here.

Link: https://lkml.kernel.org/r/49f67983-b802-8929-edab-d807f745c9ca@kernel.dk


Signed-off-by: Jens Axboe <axboe@kernel.dk>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>