Btrfs: do not warn_on io_ctl->cur in io_ctl_map_page

io_ctl_map_page is called by many functions in free-space-cache.
In most scenarios, the ->cur is not null, e.g. io_ctl_add_entry.
I think we'd better remove the warn_on here.

Signed-off-by: Wang Sheng-Hui <shhuiw@gmail.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>