selftests: vm: remove dependecy from internal kernel macros

selftests: vm: remove dependecy from internal kernel macros

The defination of swap() is used from kernel's internal header when this
test is built in source tree.  The build fails when this test is built out
of source tree as defination of swap() isn't found.  Selftests shouldn't
depend on kernel's internal header files.  They can only depend on uapi
header files.  Add the defination of swap() to fix the build error:

	gcc -Wall  -I/linux_mainline2/build/usr/include -no-pie    userfaultfd.c -lrt -lpthread -o /linux_mainline2/build/kselftest/vm/userfaultfd
	userfaultfd.c: In function `userfaultfd_stress':
	userfaultfd.c:1530:3: warning: implicit declaration of function `swap'; did you mean `swab'? [-Wimplicit-function-declaration]
	 1530 |   swap(area_src, area_dst);
	      |   ^~~~
	      |   swab
	/usr/bin/ld: /tmp/cclUUH7V.o: in function `userfaultfd_stress':
	userfaultfd.c:(.text+0x4d64): undefined reference to `swap'
	/usr/bin/ld: userfaultfd.c:(.text+0x4d82): undefined reference to `swap'
	collect2: error: ld returned 1 exit status

Link: https://lkml.kernel.org/r/20220119101531.2850400-11-usama.anjum@collabora.com
Fixes: 2c769ed7137a ("tools/testing/selftests/vm/userfaultfd.c: use swap() to make code cleaner")
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Reviewed-by: Alistair Popple <apopple@nvidia.com>
Cc: Andr Almeida <andrealmeid@collabora.com>
Cc: Darren Hart <dvhart@infradead.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Mat Martineau <mathew.j.martineau@linux.intel.com>
Cc: Matthieu Baerts <matthieu.baerts@tessares.net>
Cc: Mickal Salan <mic@digikod.net>
Cc: Minghao Chi <chi.minghao@zte.com.cn>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>