selftests/eventfd: correct test name and improve messages

selftests/eventfd: correct test name and improve messages

- Rename test from eventfd_chek_flag_cloexec_and_nonblock to
eventfd_check_flag_cloexec_and_nonblock.

- Make the RDWR‐flag comment declarative:
  “The kernel automatically adds the O_RDWR flag.”
- Update semaphore‐flag failure message to:
  “eventfd semaphore flag check failed: …”

Link: https://lkml.kernel.org/r/20250513074411.6965-1-seokwoo.chung130@gmail.com
Signed-off-by: Ryan Chung <seokwoo.chung130@gmail.com>
Reviewed-by: Wen Yang <wen.yang@linux.dev>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>