Avoid WARN() from procfs on kstat collision

Avoid WARN() from procfs on kstat collision

When we load a ZFS pool having spa_name equals to some existing kstat
we would have to create a duplicate entry, which procfs doesn't like.

For instance a ZFS pool named "zil" would have its kstat "txgs"
(module "zfs/zil") intalled under "/proc/spl/kstat/zfs/zil":
unfortunately we already have a kstat named "zil" (module "zfs")
installed in the same procfs location.

Avoid this issue by skipping the duplicate entry creation in procfs.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #628