Fix userspace memory leaks found by Clang Static Analzyer

Recently, I have been making a push to fix things that coverity found.
However, I was curious what Clang's static analyzer reported, so I ran
it and found things that coverity had missed.

• contrib/pam_zfs_key/pam_zfs_key.c: If prop_mountpoint is passed more than once, we leak memory.
• module/zfs/zcp_get.c: We leak memory on temporary properties in userspace.
• tests/zfs-tests/cmd/draid.c: On error from vdev_draid_rand(), we leak memory if best_map had been allocated by a prior iteration.
• tests/zfs-tests/cmd/mkfile.c: Memory used by the loop is not freed before program termination.

Arguably, these are all minor issues, but if we ignore them, then they
could obscure serious bugs, so we fix them.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #13955