What's the nfs_inode->mm_cred for?  The cred for the current user is assigned
to it in nfs_open (if we're writing).  Comments suggest it's so we have a cred
around to work with when we do mmap'ing.

Note that the default in the rpc code is to give you a cred for current.

Following lines in nfs_writepage_sync are interesting, somewhat typical:

	if (file)
		cred = get_rpccred(nfs_file_cred(file));
	if (!cred)
		cred = get_rpccred(NFS_I(inode)->mm_cred);

where nfs_file_cred just casts file->private_data to a cred if file is non-NULL
(the test above is superfluous).

This cred ends up being passed to rpc_call_sync (by nfs4_proc_write).