NFS: nfsacl_{encode,decode} should return signed integer

Clean up.

The nfsacl_encode() and nfsacl_decode() functions return negative
errno values, and each call site verifies that the returned value
is not negative.  Change the synopsis of both of these functions
to reflect this usage.

Document the synopsis and return values.

Reported-by: Trond Myklebust <trond.myklebust@netapp.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>