mm/truncate: add folio_unmap_invalidate() helper

mm/truncate: add folio_unmap_invalidate() helper

Add a folio_unmap_invalidate() helper, which unmaps and invalidates a
given folio. The caller must already have locked the folio. Use this
new helper in invalidate_inode_pages2_range(), rather than duplicate
the code there.

In preparation for using this elsewhere as well, have it take a gfp_t
mask rather than assume GFP_KERNEL is the right choice. This bubbles
back to invalidate_complete_folio2() as well.

Signed-off-by: Jens Axboe <axboe@kernel.dk>