gdbserver/linux: take condition out of callback in find_lwp_pid

Just a small optimization, it's not necessary to recompute lwp at each
iteration.

While at it, change the variable type to long, as ptid_t::lwp returns a
long.

Reviewed-By: Andrew Burgess <aburgess@redhat.com>
Change-Id: I181670ce1f90b59cb09ea4899367750be2ad9105