Revert r360514, to avoid unnecessary churn of the sources.

r360514 prepared the NFS code for changes to handle ext_pgs mbufs on
the receive side. However, at this time, KERN_TLS does not pass
ext_pgs mbufs up through soreceive(). As such, as this time, only
the send/build side of the NFS mbuf code needs to handle ext_pgs mbufs.
Revert r360514 since the rather extensive changes required for receive
side ext_pgs mbufs are not yet needed.
This avoids unnecessary churn of the sources.