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/sys.c

@@ -1916,10 +1916,10 @@ static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd)
 	int err;
 
 	exe = fdget(fd);
-	if (!exe.file)
+	if (!fd_file(exe))
 		return -EBADF;
 
-	inode = file_inode(exe.file);
+	inode = file_inode(fd_file(exe));
 
 	/*
 	 * Because the original mm->exe_file points to executable file, make
@@ -1927,14 +1927,14 @@ static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd)
 	 * overall picture.
 	 */
 	err = -EACCES;
-	if (!S_ISREG(inode->i_mode) || path_noexec(&exe.file->f_path))
+	if (!S_ISREG(inode->i_mode) || path_noexec(&fd_file(exe)->f_path))
 		goto exit;
 
-	err = file_permission(exe.file, MAY_EXEC);
+	err = file_permission(fd_file(exe), MAY_EXEC);
 	if (err)
 		goto exit;
 
-	err = replace_mm_exe_file(mm, exe.file);
+	err = replace_mm_exe_file(mm, fd_file(exe));
 exit:
 	fdput(exe);
 	return err;