Fix incorrect usage of `rb_fiber_scheduler_io_(p)(read|write)`.
	 (#7593)

	---
	 io_buffer.c                  |  8 ++++----
	 scheduler.c                  | 48 ++++++++++++++++++++++----------------------
	 test/fiber/test_io_buffer.rb | 33 ++++++++++++++++++++++++++++++
	 3 files changed, 61 insertions(+), 28 deletions(-)

I got the warning while running "make test-all":

	Leaked file descriptor: TestFiberIOBuffer#test_write_nonblock: 9 : #<UNIXSocket:fd 9>
	Closed file descriptor: TestFiberIOBuffer#test_read_write_blocking: 9
	Leaked file descriptor: TestFiberIOBuffer#test_timeout_after: 10 : #<UNIXSocket:fd 10>
	Closed file descriptor: TestFiberIOBuffer#test_read_nonblock: 10