selftests: introduce additional eventfd test coverage

selftests: introduce additional eventfd test coverage

Add several new test cases which assert corner cases on the eventfd
mechanism, for example, the supplied buffer is less than 8 bytes,
attempting to write a value that is too large, etc.

	./eventfd_test
	# Starting 9 tests from 1 test cases.
	#  RUN           global.eventfd_check_flag_rdwr ...
	#            OK  global.eventfd_check_flag_rdwr
	ok 1 global.eventfd_check_flag_rdwr
	#  RUN           global.eventfd_check_flag_cloexec ...
	#            OK  global.eventfd_check_flag_cloexec
	ok 2 global.eventfd_check_flag_cloexec
	#  RUN           global.eventfd_check_flag_nonblock ...
	#            OK  global.eventfd_check_flag_nonblock
	ok 3 global.eventfd_check_flag_nonblock
	#  RUN           global.eventfd_chek_flag_cloexec_and_nonblock ...
	#            OK  global.eventfd_chek_flag_cloexec_and_nonblock
	ok 4 global.eventfd_chek_flag_cloexec_and_nonblock
	#  RUN           global.eventfd_check_flag_semaphore ...
	#            OK  global.eventfd_check_flag_semaphore
	ok 5 global.eventfd_check_flag_semaphore
	#  RUN           global.eventfd_check_write ...
	#            OK  global.eventfd_check_write
	ok 6 global.eventfd_check_write
	#  RUN           global.eventfd_check_read ...
	#            OK  global.eventfd_check_read
	ok 7 global.eventfd_check_read
	#  RUN           global.eventfd_check_read_with_nonsemaphore ...
	#            OK  global.eventfd_check_read_with_nonsemaphore
	ok 8 global.eventfd_check_read_with_nonsemaphore
	#  RUN           global.eventfd_check_read_with_semaphore ...
	#            OK  global.eventfd_check_read_with_semaphore
	ok 9 global.eventfd_check_read_with_semaphore
	# PASSED: 9 / 9 tests passed.
	# Totals: pass:9 fail:0 xfail:0 xpass:0 skip:0 error:0

Link: https://lkml.kernel.org/r/20240527000200.5615-1-wen.yang@linux.dev
Signed-off-by: Wen Yang <wen.yang@linux.dev>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Andrei Vagin <avagin@google.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Dave Young <dyoung@redhat.com>
Cc: Tim Bird <tim.bird@sony.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>