[XFS] Warn on transaction in flight on read-only remount

Till VFS can correctly support read-only remount without racing,
use WARN_ON instead of BUG_ON on detecting transaction in flight
after quiescing filesystem.

Signed-off-by: Felix Blyakher <felixb@sgi.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>