nfsd41: CREATE_EXCLUSIVE4_1

Implement the CREATE_EXCLUSIVE4_1 open mode conforming to
http://tools.ietf.org/html/draft-ietf-nfsv4-minorversion1-26

This mode allows the client to atomically create a file
if it doesn't exist while setting some of its attributes.

It must be implemented if the server supports persistent
reply cache and/or pnfs.

Signed-off-by: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>