nfs: Do not convert nfs_idmap_cache_timeout to jiffies

Since commit 57e62324 ("NFS: Store the legacy idmapper result in the
keyring") nfs_idmap_cache_timeout changed units from jiffies to seconds.
Unfortunately sysctl interface was not updated accordingly.

As a effect updating /proc/sys/fs/nfs/idmap_cache_timeout with some
value will incorrectly multiply this value by HZ.
Also reading /proc/sys/fs/nfs/idmap_cache_timeout will show real value
divided by HZ.

Fixes: 57e62324 ("NFS: Store the legacy idmapper result in the keyring")
Signed-off-by: Jan Chochol <jan@chochol.info>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>