NFSD: Add nfsd4_encode_fattr4_space_total()

Refactor the encoder for FATTR4_SPACE_TOTAL into a helper. In a
subsequent patch, this helper will be called from a bitmask loop.

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