Diff reduction with upstream.  The code change is not really applicable
to FreeBSD.

Illumos issue:
    5148 zvol's DKIOCFREE holds zfsdev_state_lock too long

MFC after:	1 month


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272509 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Add a new sysctl, vfs.zfs.vol.unmap_enabled, which allows the system
administrator to toggle whether ZFS should ignore UNMAP requests.

Illumos issue:
    5149 zvols need a way to ignore DKIOCFREE

MFC after:	2 weeks


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272510 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
MFV r272499:

Illumos issue:
    5174 add sdt probe for blocked read in dbuf_read()

MFC after:	2 weeks


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272511 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f