kthread: silence bogus section mismatch warning

WARNING: kernel/built-in.o(.text+0x16910): Section mismatch:
reference to .init.text: (between 'kthreadd' and 'init_waitqueue_head')

comes because kernel/kthread.c:kthreadd() is not __init but calls
kthreadd_setup() which is __init. But this is ok, because kthreadd_setup()
is only ever called at init time, and then kthreadd() proceeds into its
"for (;;)" loop. We could mark kthreadd __init_refok, but kthreadd_setup()
with just one callsite and 4 lines in it (it's been that small since
10ab825b

) doesn't need to be a separate function at all -- so let's
just move those four lines at beginning of kthreadd() itself.

Signed-off-by: Satyam Sharma <ssatyam@cse.iitk.ac.in>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>