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/20220510203222.24246-6-shy828301@gmail.com
Signed-off-by: Yang Shi <shy828301@gmail.com>
Acked-by: Song Liu <song@kernel.org>
Acked-by: Vlastmil Babka <vbabka@suse.cz>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Song Liu <songliubraving@fb.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>