kinst: do not look for a function epilogue

kinst currently only traces functions that start and end with the usual
function prologue and epilogue respectively. Ignoring functions that do
not have an epilogue however, makes the filtering too strict, as this
means that we can not trace functions that never return (e.g
vnlru_proc()). This patch relaxes the filtering and only checks whether
the function pushes the frame pointer.

Reviewed by: markj
Approved by: markj
Differential Revision: https://reviews.freebsd.org/D41876