sunrpc: simplify rpcauth_cache_shrink_count()

It is inappropriate to use sysctl_vfs_cache_pressure here.
The sysctl is documented as: This percentage value controls
the tendency of the kernel to reclaim the memory which is used
for caching of directory and inode objects.

So, simplify result of rpcauth_cache_shrink_count() to
"return number_cred_unused;".

Signed-off-by: Kaixiong Yu <yukaixiong@huawei.com>
Reviewed-by: Kees Cook <kees@kernel.org>
Acked-by: Anna Schumaker <anna.schumaker@oracle.com>
Acked-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Joel Granados <joel.granados@kernel.org>