zfs_allow_log_key TSD key: re-work the handling of this key's value
via saved_poolname.  Build saved_poolname in a local array and then
use strdup() to save it when necessary.  The code previously allocated
space for the TSD value with kmem_alloc which doesn't seemt to play well
(accounting-wise) with strfree() (which is the function used to free
the value later on).