MFC r246631,246651,246666,246675,246678,246688:
Merge various ZFS bugfixes

MFC r246631:
Import vendor bugfixes

Illumos ZFS issues:
  3422 zpool create/syseventd race yield non-importable pool
  3425 first write to a new zvol can fail with EFBIG

MFC r246651:
Import minor type change in refcount.h header from vendor (illumos).

MFC r246666:
Import vendor ZFS bugfix fixing a problem in arc_read().

Illumos ZFS issues:
  3498 panic in arc_read(): !refcount_is_zero(&pbuf->b_hdr->b_refcnt)

MFC r246675:
Add tunable to allow block allocation on degraded vdevs.

Illumos ZFS issues:
  3507 Tunable to allow block allocation even on degraded vdevs

MFC r246678:
Import vendor bugfixes regarding SA rounding, header size and layout.
This was already partially fixed by avg.

Illumos ZFS issues:
  3512 rounding discrepancy in sa_find_sizes()
  3513 mismatch between SA header size and layout

MFC r246688 [1]:
Merge zfs_ioctl.c code that should have been merged together with ZFS v28.
Fixes several problems if working with read-only pools.

Changed code originaly introduced in onnv-gate 13061:bda0decf867b
Contains changes up to illumos-gate 13700:4bc0783f6064

PR:		kern/175897 [1]
Suggested by:	avg [1]