mm: khugepaged: make khugepaged_enter() void function

mm: khugepaged: make khugepaged_enter() void function

The most callers of khugepaged_enter() don't care about the return value.
Only dup_mmap(), anonymous THP page fault and MADV_HUGEPAGE handle the
error by returning -ENOMEM.  Actually it is not harmful for them to ignore
the error case either.  It also sounds overkilling to fail fork() and page
fault early due to khugepaged_enter() error, and MADV_HUGEPAGE does set
VM_HUGEPAGE flag regardless of the error.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Yang Shi <[email protected]>
Acked-by: Song Liu <[email protected]>
Acked-by: Vlastmil Babka <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Miaohe Lin <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Song Liu <[email protected]>
Cc: Theodore Ts'o <[email protected]>
Cc: Zi Yan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>