tmpfs: fix kernel BUG in shmem_delete_inode

tmpfs: fix kernel BUG in shmem_delete_inode

commit 14fcc23fdc78e9d32372553ccf21758a9bd56fa1 upstream

SuSE's insserve initscript ordering program hits kernel BUG at mm/shmem.c:814
on 2.6.26.  It's using posix_fadvise on directories, and the shmem_readpage
method added in 2.6.23 is letting POSIX_FADV_WILLNEED allocate useless pages
to a tmpfs directory, incrementing i_blocks count but never decrementing it.

Fix this by assigning shmem_aops (pointing to readpage and writepage and
set_page_dirty) only when it's needed, on a regular file or a long symlink.

Many thanks to Kel for outstanding bugreport and steps to reproduce it.

Reported-by: Kel Modderman <kel@otaku42.de>
Tested-by: Kel Modderman <kel@otaku42.de>
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>