Fix 'zpool add' handling of nested interior VDEVs

When replacing a faulted device which was previously handled by a spare
multiple levels of nested interior VDEVs will be present in the pool
configuration; the following example illustrates one of the possible
situations:

   NAME                          STATE     READ WRITE CKSUM
   testpool                      DEGRADED     0     0     0
     raidz1-0                    DEGRADED     0     0     0
       spare-0                   DEGRADED     0     0     0
         replacing-0             DEGRADED     0     0     0
           /var/tmp/fault-dev    UNAVAIL      0     0     0  cannot open
           /var/tmp/replace-dev  ONLINE       0     0     0
         /var/tmp/spare-dev1     ONLINE       0     0     0
       /var/tmp/safe-dev         ONLINE       0     0     0
   spares
     /var/tmp/spare-dev1         INUSE     currently in use

This is safe and allowed, but get_replication() needs to handle this
situation gracefully to let zpool add new devices to the pool.

Reviewed-by: George Melikov <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: loli10K <[email protected]>
Closes #6678 
Closes #6996 

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