kthread: fix spelling typo and grammar in comments

- `If present` -> `If present,'
- `reuturn` -> `return`
- `function exit safely` -> `function to exit safely`

Link: https://lkml.kernel.org/r/20230502090242.3037194-1-quic_pbaronia@quicinc.com


Signed-off-by: Prathu Baronia <quic_pbaronia@quicinc.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Sami Tolvanen <samitolvanen@google.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Zqiang <qiang1.zhang@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>