kernel/nsproxy: remove unnecessary guards

kernel/nsproxy: remove unnecessary guards

In free_nsproxy() and the error path of create_new_namesapces() the
put_*_ns() calls are guarded by unnecessary NULL checks.

put_pid_ns(), put_ipc_ns(), put_uts_ns(), and put_time_ns() will never
receive a NULL argument unless their namespace type is disabled, and in
this case all four become no-ops at compile time anyway. put_mnt_ns()
will never receive a null argument at any time.

This unguarded usage is in line with other call sites of put_*_ns().

Signed-off-by: Joel Savitz <jsavitz@redhat.com>
Link: https://lore.kernel.org/20250508184930.183040-2-jsavitz@redhat.com
Signed-off-by: Christian Brauner <brauner@kernel.org>