mm: khugepaged: move some khugepaged_* functions to khugepaged.c

mm: khugepaged: move some khugepaged_* functions to khugepaged.c

To reuse hugepage_vma_check() for khugepaged_enter() so that we could
remove some duplicate code.  But moving hugepage_vma_check() to
khugepaged.h needs to include huge_mm.h in it, it seems not optimal to
bloat khugepaged.h.

And the khugepaged_* functions actually are wrappers for some non-inline
functions, so it seems the benefits are not too much to keep them inline.

So move the khugepaged_* functions to khugepaged.c, any callers just need
to include khugepaged.h which is quite small.  For example, the following
patches will call khugepaged_enter() in filemap page fault path for
regular filesystems to make readonly FS THP collapse more consistent.  The
filemap.c just needs to include khugepaged.h.

Link: https://lkml.kernel.org/r/20220404200250.321455-7-shy828301@gmail.com
Signed-off-by: Yang Shi <shy828301@gmail.com>
Acked-by: Song Liu <song@kernel.org>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>