8558 lwp_create() returns EAGAIN on system with more than 80K ZFS filesystems

illumos/illumos-gate@216d7723a1a58124cf95c4950d51d5f99d3f4128
https://github.com/illumos/illumos-gate/commit/216d7723a1a58124cf95c4950d51d5f99d3f4128

https://www.illumos.org/issues/8558
  On a system with more than 80K ZFS filesystems, we've seen cases where
  lwp_create() will start to fail by returning EAGAIN. The problem being,
  for each of those 80K ZFS filesystems, a taskq will be created for each
  dataset as part of the ZIL for each dataset.
  For each of these taskq's, a kernel thread will be created which results
  in 24KB being allocated for each thread. With enough of these 24KB
  allocations, we eventually exhaust the memory region set aside for these
  allocations. Currently, segkpsize is set to a value of 2GB, which means
  we can only support about 80K filesystems; 2GB / 24KB = ~80K.
  The lwp_create() failure comes into play due to the fact that LWP
  creation also allocates 24KB from this same region of memory. Thus, if
  we've exhausted this region of memory due to the number of ZIL taskq's,
  there won't be any memory avaible to allow the call to lwp_create() to
  succeed.

Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Sebastien Roy <sebastien.roy@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Prakash Surya <prakash.surya@delphix.com>

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