selinux: check return value of sel_make_avc_files

sel_make_avc_files() might fail and return a negative errno value on
memory allocation failures. Re-add the check of the return value,
dropped in 66f8e2f0 ("selinux: sidtab reverse lookup hash table").

Reported by clang-analyzer:

    security/selinux/selinuxfs.c:2129:2: warning: Value stored to
      'ret' is never read [deadcode.DeadStores]
            ret = sel_make_avc_files(dentry);
            ^     ~~~~~~~~~~~~~~~~~~~~~~~~~~

Fixes: 66f8e2f0 ("selinux: sidtab reverse lookup hash table")
Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
[PM: description line wrapping, added proper commit ref]
Signed-off-by: Paul Moore <paul@paul-moore.com>