MFC r207670, r208130, r208131:

MFC r207670, r208130, r208131:

MFC r207670:
Introduce hardforce export option (-F) for "zpool export".
When exporting with this flag, zpool.cache remains untouched.
OpenSolaris onnv revision: 8211:32722be6ad3b

MFC r208130:
Fix perfomance problem with ZFS prefetch caching [1]
Add statistics for ZFS prefetch (sysctl kstat.zfs.misc.zfetchstats)
OpenSolaris onnv revision: 10474:0e96dd3b905a (partial)

MFC r208131:
Fix deadlock between zfs_dirent_lock and zfs_rmdir
OpenSolaris onnv revision: 11321:506b7043a14c

Reported by:	jhell@dataix.net (private e-mail) [1]
Approved by:	pjd, delphij (mentor)
Obtained from:	OpenSolaris (Bug ID: 6775357, 6859997, 6868951, 6847615)

git-svn-id: svn://svn.freebsd.org/base/stable/8@208288 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f