cifs: better instrumentation for coalesce_t2

When coalesce_t2 returns an error, have it throw a cFYI message that
explains the reason. Also rename some variables to clarify what they
represent.

Reported-and-Tested-by: Konstantinos Skarlatos <k.skarlatos@gmail.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>