nfsd: merge cookie collision fixes from ext4 tree

These changes fix readdir loops on ext4 filesystems with dir_index
turned on.  I'm pulling them from Ted's tree as I'd like to give them
some extra nfsd testing, and expect to be applying (potentially
conflicting) patches to the same code before the next merge window.

From the nfs-ext4-premerge branch of

	git://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4

Signed-off-by: J. Bruce Fields <bfields@redhat.com>