struct export_operations: adjust comments to match current members

The comments in the definition of struct export_operations don't match the
current members.

Add a comment for the 2 new functions and remove 2 comments for unused ones.

Signed-off-by: Marc Dionne <marc.c.dionne@gmail.com>
Acked-by: David Howells <dhowells@redhat.com>
Acked-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>