mm/page-writeback.c: print a warning if the vm dirtiness settings are illogical

The vm direct limit setting must be set greater than vm background limit
setting.  Otherwise print a warning to help the operator to figure out
that the vm dirtiness settings is in illogical state.

Link: http://lkml.kernel.org/r/1506592464-30962-1-git-send-email-laoar.shao@gmail.com
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>