block: don't warn when doing fsync on read-only devices

It is possible to call fsync on a read-only handle (for example, fsck.ext2
does it when doing read-only check), and this call results in kernel
warning.

The patch b089cfd9

 ("block: don't warn for flush on read-only device")
attempted to disable the warning, but it is buggy and it doesn't
(op_is_flush tests flags, but bio_op strips off the flags).

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Fixes: 721c7fc7

 ("block: fail op_is_write() requests to read-only partitions")
Cc: stable@vger.kernel.org	# 4.18
Signed-off-by: Jens Axboe <axboe@kernel.dk>