fuse_writepages_fill() calls tree_insert() with ap->num_pages = 0 which
triggers the following warning:

 WARNING: CPU: 1 PID: 17211 at fs/fuse/file.c:1728 tree_insert+0xab/0xc0 [fuse]
 RIP: 0010:tree_insert+0xab/0xc0 [fuse]
 Call Trace:
  fuse_writepages_fill+0x5da/0x6a0 [fuse]
  write_cache_pages+0x171/0x470
  fuse_writepages+0x8a/0x100 [fuse]
  do_writepages+0x43/0xe0

Fix up the warning and clean up the code around rb-tree insertion:

 - Rename tree_insert() to fuse_insert_writeback() and make it return the
   conflicting entry in case of failure

 - Re-add tree_insert() as a wrapper around fuse_insert_writeback()

 - Rename fuse_writepage_in_flight() to fuse_writepage_add() and reverse
   the meaning of the return value to mean

    + "true" in case the writepage entry was successfully added

    + "false" in case it was in-fligt queued on an existing writepage
       entry's auxiliary list or the existing writepage entry's temporary
       page updated

   Switch from fuse_find_writeback() + tree_insert() to
   fuse_insert_writeback()

 - Move setting orig_pages to before inserting/updating the entry; this may
   result in the orig_pages value being discarded later in case of an
   in-flight request

 - In case of a new writepage entry use fuse_writepage_add()
   unconditionally, only set data->wpa if the entry was added.

Fixes: 6b2fb799 ("fuse: optimize writepages search")
Reported-by: kernel test robot <rong.a.chen@intel.com>
Original-path-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>