kallsyms: provide symbol sizes in /proc/kallmodsyms

kallsyms: provide symbol sizes in /proc/kallmodsyms

For modules, we can simply extract these sizes from the module symtab; for core
kernel symbols, we must do subtraction as get_symbol_pos() did: this is now
abstracted into a new get_symbol_size().

Because /proc/kallmodsyms contains *all* symbols, where get_symbol_pos() was
normally only called for a small subset of symbols, this exercises this code
like never before, and has revealed a bug: the size of __per_cpu_end was being
returned as a ludicrously vast value, because the next symbol after
__per_cpu_end is far up the address space, in the kernel proper. Fixing this by
forcing a size of zero for __per_cpu_end is easy enough.

Signed-off-by: Nick Alcock <nick.alcock@oracle.com>