MFC r252056:
Fix destroyed ZFS pools failing to import

MFC r252059:
Fix ZFS zpool freeze (debug command) not processing due to invalid ioctl call syntax.

MFC r252060:
Fix intermittent ZFS lock panic

MFC r252061:
Switch ZFS mutex_owner macro to use sx_xholder as its now exported via sx.h

(cherry picked from commit b2b7537d513594904c16ddba20ff401854d2ed19)
MFC r252390:
Remove invalid ASSERT

(cherry picked from commit dabb037967e4a7519429defe038a956acc9b0b62)
Merge r252337 from head:

  Don't try to re-insert an already present but invalid page.

  This could happen if a thread doing a page-in loses a ZFS range lock
  race to a thread writing to the same range

  This fixes "panic: vm_page_alloc: pindex already allocated" in
  http://docs.FreeBSD.org/cgi/mid.cgi?1372165971.96049.42.camel

Submitted by:	avg

(cherry picked from commit 3d1c860017700196460cc094d4718fc9052c79e5)