disk: part_dos: Allocate at least one block size for mbr

The blk_dread() following the mbr allocation reads one block from the
device. This will lead to overflow if block size is greater than the
size of legacy_mbr. Fix this by allocating at least one block size.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Acked-by: Alexey Brodkin <abrodkin@synopsys.com>