Commit b3d4c70d added support for CLAIM_DELEG_CUR_FH to Open.
While doing this, I noticed that CLAIM_DELEG_PREV_FH support
could be added the same way.  Although I am not aware of any extant
NFSv4.1/4.2 client that uses this claim type, it seems prudent to add
support for this variant of Open to the NFSv4.1/4.2 server.

This patch does not affect mounts from extant NFSv4.1/4.2 clients,
as far as I know.

MFC after:	2 weeks