create-diff-object: create __mcount_loc section

For ftrace to be able to trace a patched function, it requires that the
__mcount_loc section contains a pointer to the function, and that the
first instruction of the function is "callq __fentry__".

Normally that work is done by the recordmcount script, but it ignores
functions that aren't in a few standard sections (.text and a few
others).
kmod/core: ftrace function_graph tracer compatibility

Steven Rostedt recommended to return "regs->ip + MCOUNT_INSN_SIZE",
which is what the function_graph tracer expects.  This fixes
function_graph tracing for a patched function.

This change also means that the function tracer will only show the
patched function once (corresponding to a trace of the original
function) rather than twice.  This is probably more in line with what a
user would expect.
Merge pull request #283 from jpoimboe/mcount

create-diff-object: create __mcount_loc section