lib/test_kmod: fix kernel-doc warnings

Fix all kernel-doc warnings in test_kmod.c:
- Mark some enum values as private so that kernel-doc is not needed
  for them
- s/thread_mutex/thread_lock/ in a struct's kernel-doc comments
- add kernel-doc info for @task_sync

test_kmod.c:67: warning: Enum value '__TEST_KMOD_INVALID' not described in enum 'kmod_test_case'
test_kmod.c:67: warning: Enum value '__TEST_KMOD_MAX' not described in enum 'kmod_test_case'
test_kmod.c:100: warning: Function parameter or member 'task_sync' not described in 'kmod_test_device_info'
test_kmod.c:134: warning: Function parameter or member 'thread_mutex' not described in 'kmod_test_device'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc:  <linux-modules@vger.kernel.org>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>