MFC r258294:
Fix ZFS deadlock when sending a snapshot which is mounted.

Approved by:	re (glebius)
Sponsored by:	Multiplay
MFC r229663: Allow to change vfs.zfs.arc_meta_limit at runtime.

- Change vfs.zfs.arc_meta_used from CTLFLAG_RDTUN to CTLFLAG_RD,
as it is not a tunable.

MFC slacker:	pjd
ZFS/GFS handling fixes