[elf][regset] simplify thread list handling in fill_note_info()

fill_note_info() iterates through the list of threads collected in
mm->core_state->dumper, allocating a struct elf_thread_core_info
instance for each and linking those into a list.

We need the entry corresponding to current to be first in the
resulting list, so the logics for list insertion is
	if it's for current or list is empty
		insert in the head
	else
		insert after the first element

However, in mm->core_state->dumper the entry for current is guaranteed
to be the first one.  Which means that both parts of condition will
be true on the first iteration and neither will be true on all subsequent
ones.

Taking the first iteration out of the loop simplifies things nicely...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>