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

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/[email protected]
Signed-off-by: Yafang Shao <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Michal Hocko <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>