nfsd: remove redundant assignments to variable len

There are a few assignments to variable len where the value is not
being read and so the assignments are redundant and can be removed.
In one case, the variable len can be removed completely. Cleans up
4 clang scan warnings of the form:

fs/nfsd/export.c:100:7: warning: Although the value stored to 'len'
is used in the enclosing expression, the value is never actually
read from 'len' [deadcode.DeadStores]

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>