Btrfs: sysfs compile fixup

Just use kobject_set_name(), that works in all kernels (I think...).
Kernels newer than 2.6.23 currently fail with:

/home/axboe/git/btrfs/btrfs-unstable/sysfs.c:188: error: unknown field
'name' specified in initializer

Signed-off-by: Chris Mason <chris.mason@oracle.com>