mm: khugepaged: add trace status description for SCAN_PAGE_HAS_PRIVATE

mm: khugepaged: add trace status description for SCAN_PAGE_HAS_PRIVATE

Commit 21abde3434f6 ("mm,thp: add read-only THP support for (non-shmem)
FS") introduced a new khugepaged scan result: SCAN_PAGE_HAS_PRIVATE, but
the corresponding description for trace events were not added.

Link: http://lkml.kernel.org/r/1574793844-2914-1-git-send-email-yang.shi@linux.alibaba.com
Fixes: 21abde3434f6 ("mm,thp: add read-only THP support for (non-shmem) FS")
Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
Cc: Song Liu <songliubraving@fb.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>