NFS: Cleanup usage of nfs_inode in fscache interface

A number of places in the fscache interface used nfs_inode when inode could
be used, simplifying the code.

Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>