By adding __set_bit and __clear_bit to the tools include directory, we
can share the bitops code.  This reveals an include loop between kernel.h,
log2.h, bitmap.h and bitops.h.  Break it the same way as the kernel does;
by moving the kernel.h include from bitops.h to bitmap.h.
Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>