mm: uninline kstrdup()

gcc inlines kstrdup into kstrdup_const() but it can very efficiently tail
call into it instead:

	$ ./scripts/bloat-o-meter ../vmlinux-000 ../obj/vmlinux
	add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-84 (-84)
	Function                                     old     new   delta
	kstrdup_const                                119      35     -84

Link: https://lkml.kernel.org/r/Y/4fDlbIhTLNLFHz@p183


Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>