Fix NFS client handling of copyin() error return

[rmacklem]

Summary

This patch is based on one by markj@, but includes
a few additions:

• Added a few m_freem() calls to avoid mbuf leaks
• Handled nfscl_doiods() return of EFAULT for both read and write.

Test Plan

Not tested by me. Hopefully markj@ will be able to
do so.