EX-5014 pcc: Limit attach queue depth

EX-5014 pcc: Limit attach queue depth

The existing async attach code does not attempt to limit
the number of async attaches that can be requested at once.
This is a problem because we could theoretically create too
many kthreads and overwhelm the system.

When the attach queue depth is exceeded, we stop allowing
new items to be queued by switching over to sync attach.

Ideally we would rebuild the attach code to generate a
queue of attach requests and have the attach thread code
pull items from the queue until it's exhausted, but that's
a much more substantial change and is left for later.

NB: This patch is incomplete - there's no way to adjust the
queue depth at runtime and there's no test for it.  Both
need to be added.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: Ib00dfb67f5245a28b722278d031ee8cdf5e190d6
Reviewed-on: https://review.whamcloud.com/47061
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Yingjin Qian <qian@ddn.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>