NFSD: Clean up nfsd4_encode_stateid()

NFSD: Clean up nfsd4_encode_stateid()

Update the encoder function name to match the type name, as is the
convention with other such encoder utility functions, and with
nfsd4_decode_stateid4().

Make the @stateid argument a const so that callers of
nfsd4_encode_stateid4() in the future can be passed const pointers
to structures.

Since the compiler is allowed to add padding to structs, use the
wire (spec-defined) size when reserving buffer space.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>