MFC r260481:
  Add MRT6_DLOG() macro for debugging.
  Reduce number of MRT6DEBUG ifdefs and fix some broken format strings.
MFC r258354: taskqueue_cancel: garbage collect a write-only variable
MFC r258351: fsx: new option to disable msync(MS_SYNC) after each write
MFC r258352: fsx: add an option to randomly call msync(MS_INVALIDATE)
MFC r258638,258642: expose zfs_flags as debug.zfs_flags r/w tunable and sysctl

Sponsored by:	HybridCluster
MFC r258647: zfsboottest: properly specify a library dependency
MFC r258648: use saner calculations in should_yield
MFC r258628: opensolaris taskq: some cosmetic changes
MFC r258630: 734 taskq_dispatch_prealloc() desired
MFC r258743: drop ZUT_OBJ
MFC r258631: MFV r247578

3581 spa_zio_taskq[ZIO_TYPE_FREE][ZIO_TASKQ_ISSUE]->tq_lock is piping hot
MFC r258632,258704: MFV r255255: 4045 zfs write throttle & i/o scheduler
performance work

Sponsored by:	HybridCluster [merge]
MFC r258633: MFV r255256: 3954 metaslabs continue to load even after
hitting zfs_mg_alloc_failure limit
MFC r258634: MFV r258376: 3964 L2ARC should always compress metadata buffers
MFC r258739: zfs mappedread_sf: assert that a page is never partially valid
MFC r258720: MFV r258665: 4347 ZPL can use dmu_tx_assign(TXG_WAIT)
MFC r259612: ttm_bo_vm_lookup_rb: actually make use of the red-black tree
MFC r260772:
  Update the pkg-stage target to be more compatible with pkg-1.2:

 - Add a release-dvd.conf pkg(8) configuration file to override
   the default FreeBSD.conf configuration.
 - Remove architecture-specific pkg-stage.conf files, consolidate,
   and move their contents to scripts/pkg-stage.sh.
 - Use 'pkg -vv' to determine the ABI, which is used as the
   cache directory.

  Prior to these changes, it would be possible for pkg-stage to fetch
  conflicting binary packages from multiple repositories.

Tested against:	head@r260522, stable/10@r260522

Sponsored by:	The FreeBSD Foundation
MFC r258744-258746: zfs: add zfs_freebsd_putpages
MFC r257037: Redefine the io provider using the SDT(9) macros

MFC slacker:	markj