nfsd: fix NFS server for ERELOOKUP

r367672 modified UFS such that certain VOPs, such as
VOP_CREATE() will intermittently return ERELOOKUP.
When this happens, the entire system call, or NFS
operation in the case of the NFS server, must be redone.

This patch adds that support to the NFS server by rolling
back the state of the NFS request arguments and NFS
reply arguments mbuf lists to the condition they were
in before the operation and then redoing the operation.

Tested by: pho
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D27875