NFSD: Replace READ* macros in nfsd4_decode_fattr()

Let's be more careful to avoid overrunning the memory that backs
the bitmap array. This requires updating the synopsis of
nfsd4_decode_fattr().

Bruce points out that a server needs to be careful to return nfs_ok
when a client presents bitmap bits the server doesn't support. This
includes bits in bitmap words the server might not yet support.

The current READ* based implementation is good about that, but that
requirement hasn't been documented.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>