We know this won't be called for child == current, so we don't need
to bother with callbacks, etc. - just do unw_init_from_blocked_task(),
unw_unwind_to_user() and do the payload of gpregs_[gs]et().  For
one register.  Which is to say, access_elf_reg().
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>