Fix memory leak in zvol_set_volsize()

Fix memory leak in zvol_set_volsize()

Move kmem_free() so it's called for every error path: this is
preferred over making `dmu_object_info_t doi` local to accommodate
older kernels with limited stacks.

Reviewed by: Boris Protopopov <bprotopopov@hotmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #6177