nstree: fix func. parameter kernel-doc warnings

nstree: fix func. parameter kernel-doc warnings

Use the correct parameter name ("__ns") for function parameter kernel-doc
to avoid 3 warnings:

Warning: include/linux/nstree.h:68 function parameter '__ns' not described in 'ns_tree_add_raw'
Warning: include/linux/nstree.h:77 function parameter '__ns' not described in 'ns_tree_add'
Warning: include/linux/nstree.h:88 function parameter '__ns' not described in 'ns_tree_remove'

Fixes: 885fc8ac0a4d ("nstree: make iterator generic")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://patch.msgid.link/20260416215429.948898-1-rdunlap@infradead.org
Signed-off-by: Christian Brauner <brauner@kernel.org>