fuse: fix fuse_send_readpages() in the syncronous read case

Buffered read in fuse normally goes via:

 -> generic_file_buffered_read()
   -> fuse_readpages()
     -> fuse_send_readpages()
       ->fuse_simple_request() [called since v5.4]

In the case of a read request, fuse_simple_request() will return a
non-negative bytecount on success or a negative error value.  A positive
bytecount was taken to be an error and the PG_error flag set on the page.
This resulted in generic_file_buffered_read() falling back to ->readpage(),
which would repeat the read request and succeed.  Because of the repeated
read succeeding the bug was not detected with regression tests or other use
cases.

The FTP module in GVFS however fails the second read due to the
non-seekable nature of FTP downloads.

Fix by checking and ignoring positive return value from
fuse_simple_request().

Reported-by: Ondrej Holy <oholy@redhat.com>
Link: https://gitlab.gnome.org/GNOME/gvfs/issues/441


Fixes: 134831e3 ("fuse: convert readpages to simple api")
Cc: <stable@vger.kernel.org> # v5.4
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>