kmemleak-test: drop __init to get better backtrace

Drop the __init on kmemleak_test_init(). With it, the storage is
reclaimed, but then the symbol isn't available for "%pS" rendering,
and the backtrace gets a bare pointer where the actual leak happened.

unreferenced object 0xffff88800a2b0800 (size 1024):
comm "modprobe", pid 413, jiffies 4294953430
hex dump (first 32 bytes):
73 02 00 00 75 01 00 68 02 00 00 01 00 00 00 04 s...u..h........
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
backtrace:
[<00000000fabad728>] kmalloc_trace+0x26/0x90
[<00000000ef738764>] 0xffffffffc02350a2
[<00000000004e5795>] do_one_initcall+0x43/0x210
[<00000000d768905e>] do_init_module+0x4a/0x210
[<0000000087135ab5>] __do_sys_finit_module+0x93/0xf0
[<000000004fcb1fa2>] do_syscall_64+0x34/0x80
[<00000000c73c8d9d>] entry_SYSCALL_64_after_hwframe+0x46/0xb0

with __init gone, that trace entry renders like:

[<00000000ef738764>] kmemleak_test_init+<offset>/<size>

Link: https://lkml.kernel.org/r/20230525174356.69711-1-jim.cromie@gmail.com
Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm,kmemleak-test.c: move kmemleak-test.c to samples dir

kmemleak-test.c is just a kmemleak test module, which also can not be used
as a built-in kernel module. Thus, i think it may should not be in mm
dir, and move the kmemleak-test.c to samples/kmemleak/kmemleak-test.c.
Fix the spelling of built-in by the way.

Signed-off-by: Hui Su <sh_def@163.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Cc: David S. Miller <davem@davemloft.net>
Cc: Rob Herring <robh@kernel.org>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Steven Rostedt (VMware) <rostedt@goodmis.org>
Cc: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Cc: Divya Indi <divya.indi@oracle.com>
Cc: Tomas Winkler <tomas.winkler@intel.com>
Cc: David Howells <dhowells@redhat.com>
Link: https://lkml.kernel.org/r/20200925183729.GA172837@rlk
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>