mm: huge_memory: debugfs for file-backed THP split

mm: huge_memory: debugfs for file-backed THP split

Further extend <debugfs>/split_huge_pages to accept
"<path>,<pgoff_start>,<pgoff_end>" for file-backed THP split tests since
tmpfs may have file backed by THP that mapped nowhere.

Update selftest program to test file-backed THP split too.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Zi Yan <[email protected]>
Suggested-by: Kirill A. Shutemov <[email protected]>
Reviewed-by: Yang Shi <[email protected]>
Cc: "Kirill A . Shutemov" <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: Sandipan Das <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Mika Penttila <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>