rtld-elf: Avoid unnecessary lock_restart_for_upgrade() calls

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