Revert "9p: Enable multipage folios"

This reverts commit 1325e4a9.

using multipage folios apparently break some madvise operations like
MADV_PAGEOUT which do not reliably unload the specified page anymore,

Revert the patch until that is figured out.

Reported-by: Andrii Nakryiko <andrii@kernel.org>
Fixes: 1325e4a9 ("9p: Enable multipage folios")
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>