LU-12018 quota: do not start a thread under memory pressure

LU-12018 quota: do not start a thread under memory pressure

this leads to a deadlock as kthreadd creating new threads
can get stuck waiting for memory as well:

PID: 2 TASK: ffff88015d1e0fb0 CPU: 3 COMMAND: "kthreadd"

Change-Id: I88f14da24ea64dcc02a9fd1f4a9c03f5771f8fda
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/34328
Tested-by: Jenkins
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>