Merge tag 'pull-stable-struct_fd' into origin

struct fd layout change (and conversion to accessor helpers)

[textual conflict in fs/coda/inode.c goes with mainline version;
non-textual conflicts in fs/namespace.c and fs/xfs/xfs_exchrange.c
fixed by replacing if (!f.file) with if (fd_empty(f)) and
other f.file instances with fd_file(f)]

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>