Fix 3-way deadlock that can happen because of ZFS and vnode lock
order reversal.

thread0 (vfs_fhtovp) thread1 (vop_getattr) thread2 (zfs_recv)

			vn_lock

rrw_enter_read

						rrw_enter_write (hangs)
			rrw_enter_read (hangs)

vn_lock (hangs)

Submitted by: Attila Nagy <bra@fsn.hu>
MFC after: 3 days