NFS: Clean up nfs_refresh_inode() and nfs_post_op_update_inode()

Try to avoid taking and dropping the inode->i_lock more than once. Do so by
moving the code in nfs_refresh_inode() that needs to be done under the
spinlock into a function nfs_refresh_inode_locked(), and then having both
nfs_refresh_inode() and nfs_post_op_update_inode() call it directly.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>