dio: add __blockdev_direct_IO_bdev()

dio: add __blockdev_direct_IO_bdev()

Previous patches refactored __blockdev_direct_IO() to call helper
functions while iterating over the user's iovec.  This adds a
__blockdev_direct_IO() which is the same except that it iterates over
the pages in a bio_vec instead of user addresses in an iovec.

The trick here is to initialize the dio state so that do_direct_IO()
consumes the pages we provide and never tries to map user pages.  This
is done by making sure that final_block_in_request covers the page that
we set in the dio.  do_direct_IO() will return before running out of
pages.

The caller is responsible for dirtying these pages, if needed.  We add
an option to the dio struct that makes sure we only dirty pages when
we're operating on iovecs of user addresses.

Signed-off-by: Zach Brown <zach.brown@oracle.com>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>