kernel: fix TOCTTOU issue in k_thread_name_set

Previously, a racing write to the provided string could result
in up to CONFIG_THREAD_MAX_NAME_LEN-2 bytes after the end
of user-accessible memory being leaked into the thread name.

For now, make a temporary copy. In an ideal world this could
copy directly from userspace into the thread name, but that
violates the current vrfy / impl split.

Signed-off-by: James Harris <james.harris@intel.com>