fuse: remove tmp folio for writebacks and internal rb tree

In the current FUSE writeback design (see commit 3be5a52b
("fuse: support writable mmap")), a temp page is allocated for every
dirty page to be written back, the contents of the dirty page are copied
over to the temp page, and the temp page gets handed to the server to
write back.

This is done so that writeback may be immediately cleared on the dirty
page, and this in turn is done in order to mitigate the following
deadlock scenario that may arise if reclaim waits on writeback on the
dirty page to complete:
* single-threaded FUSE server is in the middle of handling a request
  that needs a memory allocation
* memory allocation triggers direct reclaim
* direct reclaim waits on a folio under writeback
* the FUSE server can't write back the folio since it's stuck in
  direct reclaim

With a recent change that added AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM and
mitigates the situation described above, FUSE writeback does not need
to use temp pages if it sets AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM on its
inode mappings.

This commit sets AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM on the inode
mappings and removes the temporary pages + extra copying and the internal
rb tree.

fio benchmarks --
(using averages observed from 10 runs, throwing away outliers)

Setup:
sudo mount -t tmpfs -o size=30G tmpfs ~/tmp_mount
 ./libfuse/build/example/passthrough_ll -o writeback -o max_threads=4 -o source=~/tmp_mount ~/fuse_mount

fio --name=writeback --ioengine=sync --rw=write --bs={1k,4k,1M} --size=2G
--numjobs=2 --ramp_time=30 --group_reporting=1 --directory=/root/fuse_mount

        bs =  1k          4k            1M
Before  351 MiB/s     1818 MiB/s     1851 MiB/s
After   341 MiB/s     2246 MiB/s     2685 MiB/s
% diff        -3%          23%         45%

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Reviewed-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>