SUNRPC: Inline xdr_commit_encode

Micro-optimization: For xdr_commit_encode call sites in
net/sunrpc/xdr.c, eliminate the extra calling sequence.  On my
client, this change saves about a microsecond for every 30 calls
to xdr_reserve_space().

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