btrfs: keep private struct on stack for sync reads in btrfs_encoded_read_regular_fill_pages()

Only allocate the btrfs_encoded_read_private structure for asynchronous
(io_uring) mode.

There's no need to allocate an object from slab in the synchronous mode. In
such a case stack can be happily used as it used to be before 68d3b27e
("btrfs: move priv off stack in btrfs_encoded_read_regular_fill_pages()")
which was a preparation for the async mode.

While at it, fix the comment to reflect the atomic => refcount change in
d2966269 ("btrfs: fix use-after-free waiting for encoded read endios").

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Daniel Vacek <neelx@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>