9p: treat read return values of 0 as EOF

[Barret Rhoden <brho@google.com>]

If your 9p server shares a synthetic file system such as sysfs, the file
size from stat often disagrees with the actual file size.

Treat non-error reads of 0 as EOF, except in the case where the request
was for 0 bytes.

Tested:
	git@github.com:hugelgupf/p9.git, served "/" on 10.0.2.2

	bash-5.3# mount -t 9p -o trans=tcp,port=12345,cache=none 10.0.2.2 /tmp/foo
	bash-5.3# cat /tmp/foo/sys/devices/system/cpu/online
	0-47

Fixes: eb497943fa21 ("9p: Convert to using the netfs helper lib to do reads and caching")
Signed-off-by: Barret Rhoden <brho@google.com>
---
 fs/9p/vfs_addr.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/9p/vfs_addr.c b/fs/9p/vfs_addr.c
index 1ac0b3dcc077..df420b061664 100644
--- a/fs/9p/vfs_addr.c
+++ b/fs/9p/vfs_addr.c
@@ -73,6 +73,7 @@ static void v9fs_issue_read(struct netfs_io_subrequest *subreq)
 	char *target;
 	unsigned long long pos = subreq->start + subreq->transferred;
 	int total = 0, err, len, n;
+	size_t to_read = iov_iter_count(&subreq->io_iter);
 
 	if (S_ISLNK(rreq->inode->i_mode)) {
 		/* p9_client_readlink() must not be called for legacy protocols
@@ -105,6 +106,8 @@ static void v9fs_issue_read(struct netfs_io_subrequest *subreq)
 		__set_bit(NETFS_SREQ_CLEAR_TAIL, &subreq->flags);
 	if (pos + total >= i_size_read(rreq->inode))
 		__set_bit(NETFS_SREQ_HIT_EOF, &subreq->flags);
+	if (!err && !total && to_read)
+		__set_bit(NETFS_SREQ_HIT_EOF, &subreq->flags);
 	if (!err && total) {
 		subreq->transferred += total;
 		__set_bit(NETFS_SREQ_MADE_PROGRESS, &subreq->flags);
-- 
2.55.0.rc0.799.gd6f94ed593-goog