rtld-elf: Avoid unnecessary lock_restart_for_upgrade() calls

In order to atomically upgrade the rtld bind lock, load_filtees() may
trigger a longjmp back to _rtld_bind() so that the binding can be done
with the write lock held. However, the write lock is only needed when
filtee objects haven't already been loaded, so move the
lock_restart_for_upgrade() call to avoid unnecessary lock upgrades when
a filtee is defined.

Reviewed by: kib
Tested by: brooks
MFC after: 1 week
Sponsored by: Innovate UK

(cherry picked from commit e7951d0b04e6464b37264b8166b1e9ce368a9f1d)