cifs: Fix unbuffered read

cifs: Fix unbuffered read

If read() is done in an unbuffered manner, such that, say,
cifs_strict_readv() goes through cifs_user_readv() and thence
__cifs_readv(), it doesn't recognise the EOF and keeps indicating to
userspace that it returning full buffers of data.

This is due to ctx->iter being advanced in cifs_send_async_read() as the
buffer is split up amongst a number of rdata objects.  The iterator count
is then used in collect_uncached_read_data() in the non-DIO case to set the
total length read - and thus the return value of sys_read().  But since the
iterator normally gets used up completely during splitting, ctx->total_len
gets overridden to the full amount.

However, prior to that in collect_uncached_read_data(), we've gone through
the list of rdatas and added up the amount of data we actually received
(which we then throw away).

Fix this by removing the bit that overrides the amount read in the non-DIO
case and just going with the total added up in the aforementioned loop.

This was observed by mounting a cifs share with multiple channels, e.g.:

	mount //192.168.6.1/test /test/ -o user=shares,pass=...,max_channels=6

and then reading a 1MiB file on the share:

	strace cat /xfstest.test/1M  >/dev/null

Through strace, the same data can be seen being read again and again.

Fixes: d08089f649a0 ("cifs: Change the I/O paths to use an iterator rather than a page list")
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Paulo Alcantara (SUSE) <pc@manguebit.com>
cc: Jérôme Glisse <jglisse@redhat.com>
cc: Long Li <longli@microsoft.com>
cc: Enzo Matsumiya <ematsumiya@suse.de>
cc: Shyam Prasad N <nspmangalore@gmail.com>
cc: Rohith Surabattula <rohiths.msft@gmail.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-cifs@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>