error-injection: Consolidate override function definition

The function override_function_with_return() is defined separately for
each architecture and every architecture's definition is almost same
with each other.  E.g. x86 and powerpc both define function in its own
asm/error-injection.h header and override_function_with_return() has
the same definition, the only difference is that x86 defines an extra
function just_return_func() but it is specific for x86 and is only used
by x86's override_function_with_return(), so don't need to export this
function.

This patch consolidates override_function_with_return() definition into
asm-generic/error-injection.h header, thus all architectures can use the
common definition.  As result, the architecture specific headers are
removed; the include/linux/error-injection.h header also changes to
include asm-generic/error-injection.h header rather than architecture
header, furthermore, it includes linux/compiler.h for successful
compilation.

Reviewed-by: Masami Hiramat...