mm: make madvise(MADV_WILLNEED) support swap file prefetch

mm: make madvise(MADV_WILLNEED) support swap file prefetch

Make madvise(MADV_WILLNEED) support swap file prefetch.  If memory is
swapout, this syscall can do swapin prefetch.  It has no impact if the
memory isn't swapout.

[[email protected]: fix CONFIG_SWAP=n build]
[[email protected]: fix BUG on madvise early failure]
Signed-off-by: Shaohua Li <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Rik van Riel <[email protected]>
Signed-off-by: Sasha Levin <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>