FAT: initialize all fields in cur_part_info, simplify init

cur_part_info.{name,type} are strings. So, we don't need to memset()
the entire thing, just put the NULL-termination in the first byte.

Add missing initialization of the bootable and uuid fields.

None of these fields are actually used by fat.c. However, since it
stores the entire disk_partition_t, we should make sure that all fields
are valid.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>