selinux: clarify return code in filename_trans_read_helper_compat()

selinux: clarify return code in filename_trans_read_helper_compat()

For the "conflicting/duplicate rules" branch in
filename_trans_read_helper_compat() the Smatch static checker reports:

    security/selinux/ss/policydb.c:1953 filename_trans_read_helper_compat()
    warn: missing error code 'rc'

While the value of rc will already always be zero here, it is not
obvious that it's the case and that it's the intended return value
(Smatch expects rc to be assigned within 5 lines from the goto).
Therefore, add an explicit assignment just before the goto to make the
intent more clear and the code less error-prone.

Fixes: c3a276111ea2 ("selinux: optimize storage of filename transitions")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/selinux/722b90c4-1f4b-42ff-a6c2-108ea262bd10@moroto.mountain/
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>