Remove incorrect free() in zfs_get_pci_slots_sys_path()

Coverity found this. We attempted to free tmp, which is a pointer to a
string that should be freed by the caller.

Reviewed-by: Neal Gompa <ngompa@datto.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #13864