The recent addition of linux/bitops.h inside linux/kernel.h has
created a circular dependency:

asm/system.h -> linux/kernel.h -> linux/bitops.h -> asm/bitops.h
 -> asm/system.h

which results in an unbuildable kernel.  We appear not to need
linux/kernel.h in asm/system.h, but do need linux/linkage.h, so
make this the case.