NFSD: Clean up nfsd_symlink()

The pointer dentry is assigned a value that is never read, the
assignment is redundant and can be removed.

Cleans up clang-scan warning:
fs/nfsd/nfsctl.c:1231:2: warning: Value stored to 'dentry' is
never read [deadcode.DeadStores]
       dentry = ERR_PTR(ret);

No need to initialize "int ret = -ENOMEM;" either.

These are vestiges of nfsd_mkdir(), from whence I copied
nfsd_symlink().

Reported-by: Colin Ian King <colin.i.king@gmail.com>
Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>