afs: Fix afs_store_data() to set mtime in new operation descriptor

Fix afs_store_data() so that it sets the mtime in the new operation
descriptor otherwise the mtime on the server gets set to 0 when a write is
stored to the server.

Fixes: e49c7b2f ("afs: Build an abstraction around an "operation" concept")
Reported-by: Dave Botsch <botsch@cnf.cornell.edu>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>