Use vmem_free() in dfl_free() and add dfl_alloc()

Use vmem_free() in dfl_free() and add dfl_alloc()

This change was lost, somehow, in e5f9a9a.  Since the arrays can be
rather large, they need to be allocated with vmem_zalloc() via dfl_alloc()
and freed with vmem_free() via dfl_free().

The new dfl_alloc() function should be used to allocate object of type
dkioc_free_list_t in order that they're allocated from vmem.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Nikolay Borisov <kernel@kyup.com>
Closes #543