We have a check that setting a tag on a single entry at root succeeds,
but we were missing a check that clearing a tag on that same entry also
succeeds.

Link: http://lkml.kernel.org/r/1481667692-14500-4-git-send-email-mawilcox@linuxonhyperv.comSigned-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>