btrfs: limit async_work allocation and worker func duration

Problem statement: unprivileged user who has read-write access to more than
one btrfs subvolume may easily consume all kernel memory (eventually
triggering oom-killer).

Reproducer (./mkrmdir below essentially loops over mkdir/rmdir):

[root@kteam1 ~]# cat prep.sh

DEV=/dev/sdb
mkfs.btrfs -f $DEV
mount $DEV /mnt
for i in `seq 1 16`
do
	mkdir /mnt/$i
	btrfs subvolume create /mnt/SV_$i
	ID=`btrfs subvolume list /mnt |grep "SV_$i$" |cut -d ' ' -f 2`
	mount -t btrfs -o subvolid=$ID $DEV /mnt/$i
	chmod a+rwx /mnt/$i
done

[root@kteam1 ~]# sh prep.sh

[maxim@kteam1 ~]$ for i in `seq 1 16`; do ./mkrmdir /mnt/$i 2000 2000 & done

[root@kteam1 ~]# for i in `seq 1 4`; do grep "kmalloc-128" /proc/slabinfo | grep -v dma; sleep 60; done
kmalloc-128        10144  10144    128   32    1 : tunables    0    0    0 : slabdata    317    317      0
kmalloc-128       9992352 9992352    128   32    1 : tunables    0    0    0 : slabdata 312261 312261      0
kmalloc-12...