block: bump BLK_DEF_MAX_SECTORS to 2560

A value of 2560 (1280k) will accommodate a 10-data-disk stripe
write with chunk size 128k.  In the testing I've done using
iozone, fio, and aio-stress across a number of different storage
devices, a value of 1280 does not show a big performance
difference from 512, but will hopefully help software RAID
setups using SATA disks, as reported by Christoph.

NOTE: drivers/block/aoe/aoeblk.c sets its own max_hw_sectors_kb to
BLK_DEF_MAX_SECTORS.  So, this patch essentially changes aeoblk to
Use a larger maximum sector size, and I did not test this.

Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>