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>

include/linux/file.h

@@ -42,10 +42,12 @@ struct fd {
 #define FDPUT_FPUT       1
 #define FDPUT_POS_UNLOCK 2
 
+#define fd_file(f) ((f).file)
+
 static inline void fdput(struct fd fd)
 {
 	if (fd.flags & FDPUT_FPUT)
-		fput(fd.file);
+		fput(fd_file(fd));
 }
 
 extern struct file *fget(unsigned int fd);
@@ -79,7 +81,7 @@ static inline struct fd fdget_pos(int fd)
 static inline void fdput_pos(struct fd f)
 {
 	if (f.flags & FDPUT_POS_UNLOCK)
-		__f_unlock_pos(f.file);
+		__f_unlock_pos(fd_file(f));
 	fdput(f);
 }