This disables the "if needed, write the good copy back before the read
is completed" part of the read sequence for read-only mounts.

Cc: Jan Schmidt <list.btrfs@jan-o-sch.net>
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>
Signed-off-by: NJosef Bacik <jbacik@fusionio.com>
Signed-off-by: NChris Mason <chris.mason@fusionio.com>