spl: Use a clearer name for the user namespace fd

This fd has nothing to do with cleanup, that's just the name of the
field in zfs_cmd_t that was used to pass it to the kernel.

Call it what it is, an fd for a user namespace.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Signed-off-by: Ryan Moeller <freqlabs@FreeBSD.org>
Closes #13554