nfsd: remove redundant assignment to variable len

Variable len is being assigned a value zero and this is never
read, it is being re-assigned later. The assignment is redundant
and can be removed.

Cleans up clang scan-build warning:
fs/nfsd/nfsctl.c:636:2: warning: Value stored to 'len' is never read

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>