fix a bug in ZFS mirror code for handling multiple DVAa

The bug was introduced in r256956 "Improve ZFS N-way mirror read
performance".
The code in vdev_mirror_dva_select erroneously considers already
tried DVAs for the next attempt.  Thus, it is possible that a failing DVA
would be retried forever.
As a secondary effect, if the attempts fail with checksum error, then
checksum error reports are accumulated until the original request
ultimately fails or succeeds.  But because retrying is going on indefinitely
the cheksum reports accumulation will effectively be a memory leak.

Reviewed by:	gibbs
MFC after:	13 days
Sponsored by:	HybridCluster
zfs_deleteextattr: name buffer from namei is needed by zfs_rename

If we prematurely free the name buffer and it gets quickly recycled,
then zfs_rename may see data from another lookup or even unmapped memory
via cn_nameptr.

MFC after:	6 days
Sponsored by:	HybridCluster
Fix refcount leak on netinet ifa.

Reviewed by:	glebius
MFC after:	2 weeks
Sponsored by:	Yandex LLC