fs/direct-io.c: keep dio_warn_stale_pagecache() when CONFIG_BLOCK=n

This helper prints warning if direct I/O write failed to invalidate cache,
and set EIO at inode to warn usersapce about possible data corruption.

See also commit 5a9d929d

 ("iomap: report collisions between directio
and buffered writes to userspace").

Direct I/O is supported by non-disk filesystems, for example NFS.  Thus
generic code needs this even in kernel without CONFIG_BLOCK.

Link: http://lkml.kernel.org/r/157270038074.4812.7980855544557488880.stgit@buzz
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>