libceph: change from BUG to WARN for __remove_osd() asserts

commit cc9f1f518cec079289d11d732efa490306b1ddad upstream.

No reason to use BUG_ON for osd request list assertions.

Signed-off-by: Ilya Dryomov <idryomov@redhat.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>