knfsd: set the response bitmask for NFS4_CREATE_EXCLUSIVE

RFC 3530 says:

 If the server uses an attribute to store the exclusive create verifier, it
 will signify which attribute by setting the appropriate bit in the attribute
 mask that is returned in the results.

Linux uses the atime and mtime to store the verifier, but sends a zeroed out
bitmask back to the client.  This patch makes sure that we set the correct
bits in the bitmask in this situation.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Cc: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>