netfs: Fix non-contiguous donation between completed reads

When a read subrequest finishes, if it doesn't have sufficient coverage to
complete the folio(s) covering either side of it, it will donate the excess
coverage to the adjacent subrequests on either side, offloading
responsibility for unlocking the folio(s) covered to them.

Now, preference is given to donating down to a lower file offset over
donating up because that check is done first - but there's no check that
the lower subreq is actually contiguous, and so we can end up donating
incorrectly.

The scenario seen[1] is that an 8MiB readahead request spanning four 2MiB
folios is split into eight 1MiB subreqs (numbered 1 through 8).  These
terminate in the order 1,6,2,5,3,7,4,8.  What happens is:

	- 1 donates to 2
	- 6 donates to 5
	- 2 completes, unlocking the first folio (with 1).
	- 5 completes, unlocking the third folio (with 6).
	- 3 donates to 4
	- 7 donates to 4 incorrectly
	- 4 completes, unlocking the second folio (with 3), but can't use
	  the excess from 7.
	- 8 donates to 4, also incorrectly.

Fix this by preventing downward donation if the subreqs are not contiguous
(in the example above, 7 donates to 4 across the gap left by 5 and 6).

Reported-by: Shyam Prasad N <nspmangalore@gmail.com>
Closes: https://lore.kernel.org/r/CANT5p=qBwjBm-D8soFVVtswGEfmMtQXVW83=TNfUtvyHeFQZBA@mail.gmail.com/


Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/526707.1733224486@warthog.procyon.org.uk/ [1]
Link: https://lore.kernel.org/r/20241213135013.2964079-3-dhowells@redhat.com


cc: Steve French <sfrench@samba.org>
cc: Paulo Alcantara <pc@manguebit.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-cifs@vger.kernel.org
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>