selftests: ublk: support non-O_DIRECT backing files

selftests: ublk: support non-O_DIRECT backing files

A subsequent commit will add support for using a backing file to store
integrity data. Since integrity data is accessed in intervals of
metadata_size, which may be much smaller than a logical block on the
backing device, direct I/O cannot be used. Add an argument to
backing_file_tgt_init() to specify the number of files to open for
direct I/O. The remaining files will use buffered I/O. For now, continue
to request direct I/O for all the files.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>