nfsd: fix supported attributes for acl & labels

Oops--in 916d2d84 I moved some constants into an array for
convenience, but here I'm accidentally writing to that array.

The effect is that if you ever encounter a filesystem lacking support
for ACLs or security labels, then all queries of supported attributes
will report that attribute as unsupported from then on.

Fixes: 916d2d84 "nfsd: clean up supported attribute handling"
Signed-off-by: J. Bruce Fields <bfields@redhat.com>