module: Make find_symbol return a struct kernel_symbol

Impact: Cleanup, internal API change

Ksplice needs access to the kernel_symbol structure in order to support
modifications to the exported symbol table.

Cc: Anders Kaseorg <andersk@mit.edu>
Cc: Jeff Arnold <jbarnold@mit.edu>
Signed-off-by: Tim Abbott <tabbott@mit.edu>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> (bugfix and style)