introduce fd_file(), convert all accessors to it.

For any changes of struct fd representation we need to
turn existing accesses to fields into calls of wrappers.
Accesses to struct fd::flags are very few (3 in linux/file.h,
1 in net/socket.c, 3 in fs/overlayfs/file.c and 3 more in
explicit initializers).
	Those can be dealt with in the commit converting to
new layout; accesses to struct fd::file are too many for that.
	This commit converts (almost) all of f.file to
fd_file(f).  It's not entirely mechanical ('file' is used as
a member name more than just in struct fd) and it does not
even attempt to distinguish the uses in pointer context from
those in boolean context; the latter will be eventually turned
into a separate helper (fd_empty()).

	NOTE: mass conversion to fd_empty(), tempting as it
might be, is a bad idea; better do that piecewise in commit
that convert from fdget...() to CLASS(...).

[conflicts in fs/fhandle.c, kernel/bpf/syscall.c, mm/memcontrol.c
caught by git; fs/stat.c one got caught by git grep]
[fs/xattr.c conflict]

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

kernel/watch_queue.c

@@ -666,8 +666,8 @@ struct watch_queue *get_watch_queue(int fd)
 	struct fd f;
 
 	f = fdget(fd);
-	if (f.file) {
-		pipe = get_pipe_info(f.file, false);
+	if (fd_file(f)) {
+		pipe = get_pipe_info(fd_file(f), false);
 		if (pipe && pipe->watch_queue) {
 			wqueue = pipe->watch_queue;
 			kref_get(&wqueue->usage);