NFS: change access cache to use 'struct cred'.

Rather than keying the access cache with 'struct rpc_cred',
use 'struct cred'.  Then use cred_fscmp() to compare
credentials rather than comparing the raw pointer.

A benefit of this approach is that in the common case we avoid the
rpc_lookup_cred_nonblock() call which can be slow when the cred cache is large.
This also keeps many fewer items pinned in the rpc cred cache, so the
cred cache is less likely to get large.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>