selftests/mm: build with -O2

The mm kselftests are currently built with no optimisation (-O0).  It's
unclear why, and besides being obviously suboptimal, this also prevents
the pkeys tests from working as intended.  Let's build all the tests with
-O2.

[kevin.brodsky@arm.com: silence unused-result warnings]
  Link: https://lkml.kernel.org/r/20250107170110.2819685-1-kevin.brodsky@arm.com
Link: https://lkml.kernel.org/r/20241209095019.1732120-6-kevin.brodsky@arm.com
Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Cc: Aruna Ramakrishna <aruna.ramakrishna@oracle.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Joey Gouly <joey.gouly@arm.com>
Cc: Keith Lucas <keith.lucas@oracle.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/mm/Makefile

@@ -33,9 +33,16 @@ endif
 # LDLIBS.
 MAKEFLAGS += --no-builtin-rules
 
-CFLAGS = -Wall -I $(top_srcdir) $(EXTRA_CFLAGS) $(KHDR_INCLUDES) $(TOOLS_INCLUDES)
+CFLAGS = -Wall -O2 -I $(top_srcdir) $(EXTRA_CFLAGS) $(KHDR_INCLUDES) $(TOOLS_INCLUDES)
 LDLIBS = -lrt -lpthread -lm
 
+# Some distributions (such as Ubuntu) configure GCC so that _FORTIFY_SOURCE is
+# automatically enabled at -O1 or above. This triggers various unused-result
+# warnings where functions such as read() or write() are called and their
+# return value is not checked. Disable _FORTIFY_SOURCE to silence those
+# warnings.
+CFLAGS += -U_FORTIFY_SOURCE
+
 KDIR ?= /lib/modules/$(shell uname -r)/build
 ifneq (,$(wildcard $(KDIR)/Module.symvers))
 ifneq (,$(wildcard $(KDIR)/include/linux/page_frag_cache.h))