kselftest/arm64: Fix check_buffer_fill test

The check_buffer_fill test reports the error below because the test
plan is not declared correctly:

# Planned tests != run tests (0 != 20)

Fix the test adding the correct test plan declaration.

Fixes: e9b60476bea0 ("kselftest/arm64: Add utilities and a test to validate mte memory")
Signed-off-by: Vincenzo Frascino <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Cc: Catalin Marinas <[e-mail]>
Cc: Will Deacon <[e-mail]>
Cc: Gabor Kertesz <[e-mail]>
Cc: Amit Daniel Kachhap <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Will Deacon <[e-mail]>

tools/testing/selftests/arm64/mte/check_buffer_fill.c

···
 	/* Register SIGSEGV handler */
 	mte_register_signal(SIGSEGV, mte_default_handler);
 
+	/* Set test plan */
+	ksft_set_plan(20);
+
 	/* Buffer by byte tests */
 	evaluate_test(check_buffer_by_byte(USE_MMAP, MTE_SYNC_ERR),
 	"Check buffer correctness by byte with sync err mode and mmap memory\n");