mpathpersist: fix registration rollback issue

mpathpersist: fix registration rollback issue

When mpathpersist tries to rollback the registration, it copies
the SARK to the RK, and clears the SARK. However, it repeated this step
for each thread. This means that the first rollback thread correctly
had the RK set to the SARK used during registration. However, if more
than one registration needed to be rolled back, later threads would have
both the RK and SARK cleared. This commit fixes that by only copying and
clearing the SARK once.

Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>