tmpfs: simplify unuse and writepage

shmem_unuse_inode() and shmem_writepage() contain a little code to cope
with pages inserted independently into the filecache, probably by a
filesystem stacked on top of tmpfs, then fed to its ->readpage() or
->writepage().

Unionfs was indeed experimenting with working in that way three years ago,
but I find no current examples: nowadays the stacking filesystems use vfs
interfaces to the lower filesystem.

It's now illegal: remove most of that code, adding some WARN_ON_ONCEs.

Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Erez Zadok <ezk@fsl.cs.sunysb.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>