nfsd: nfs4xdr.c do-while is not a compound statement

The WRITEMEM macro produces sparse warnings of the form:
fs/nfsd/nfs4xdr.c:2668:2: warning: do-while statement is not a compound statement

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Cc: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>