btrfs: restore the thread_pool= behavior in remount for the end I/O workqueues

Commit d7b9416f ("btrfs: remove btrfs_end_io_wq") converted the read
and I/O handling from btrfs_workqueues to Linux workqueues, and as part
of that lost the code to apply the thread_pool= based max_active limit
on remount.  Restore it.

Fixes: d7b9416f ("btrfs: remove btrfs_end_io_wq")
CC: stable@vger.kernel.org # 6.0+
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>