NFSD: Fix nfsd4_encode_fattr4() crasher

Ensure that args.acl is initialized early. It is used in an
unconditional call to kfree() on the way out of
nfsd4_encode_fattr4().

Reported-by: Scott Mayhew <smayhew@redhat.com>
Fixes: 83ab8678 ("NFSD: Add struct nfsd4_fattr_args")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>