fs: guard_bio_eod() needs to consider partitions

guard_bio_eod() needs to look at the partition capacity, not just the
capacity of the whole device, when determining if truncation is
necessary.

[   60.268688] attempt to access beyond end of device
[   60.268690] unknown-block(9,1): rw=0, want=67103509, limit=67103506
[   60.268693] buffer_io_error: 2 callbacks suppressed
[   60.268696] Buffer I/O error on dev md1p7, logical block 4524305, async page read

Fixes: 74d46992 ("block: replace bi_bdev with a gendisk pointer and partitions index")
Cc: stable@vger.kernel.org # v4.13
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Greg Edwards <gedwards@ddn.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>