getnewvnode_reserve helps to avoid "recursing" back into zfs code
via getnewvnode when that latter needs to reclaim some vnodes.
zfs code may hold a number of locks around getnewvnode and doesn't
expect any recursion to happen on those locks, because that never
happens in solaris.
I believe that this change also eleiminates a need for the delayed
znode destruction via the taskqueue.
Many thanks to kib for devising getnewvnode_reserve.
Reported by: flo
Tested by: bapt, kwm, swills
MFC after: 2 weeks
X-MFC after: r241556
edb085b8ab